<PAGE>   1
                       SECURITIES AND EXCHANGE COMMISSION
                              WASHINGTON, DC 20549


                                   FORM 11-K


                 ANNUAL REPORT PURSUANT TO SECTION 15(d) OF THE
                        SECURITIES EXCHANGE ACT OF 1934


(Mark One)
    [X] Annual report pursuant to Section 15(d) of the Securities Exchange Act
        of 1934
(No fee required, effective October 7, 1996)

    For the fiscal year ended              DECEMBER 31, 1999
                              --------------------------------------------------

                                       Or

    [ ] Transition report pursuant to Section 15(d) of the Securities Exchange 
        Act of 1934
(No fee required)

    For the transition period from                      to  
                                   -------------------      --------------------

    Commission file number                            1-12317
                           -----------------------------------------------------

    A. Full title of the plan and the address of the plan, if different from 
that of the issuer named below     NATIONAL-OILWELL RETIREMENT AND THRIFT PLAN
                               -------------------------------------------------

    B. Name of issuer of the securities held pursuant to the plan and the 
address of its principal executive office  

    NATIONAL-OILWELL, INC.
    ----------------------------------------------------------------------------
    10000 RICHMOND AVENUE
    ----------------------------------------------------------------------------
    4th FLOOR
    ----------------------------------------------------------------------------
    HOUSTON, TEXAS 77042-4200
    ----------------------------------------------------------------------------

<PAGE>   2




                              REQUIRED INFORMATION


1.       Not Applicable.

2.       Not Applicable.

3.       Not Applicable.

4.       The National-Oilwell Retirement and Thrift Plan (the Plan) is subject
         to the requirements of the Employee Retirement Income Security Act of
         1974 (ERISA). Attached hereto is a copy of the most recent financial
         statements and schedules of the Plan prepared in accordance with the
         financial reporting requirements of ERISA.


<PAGE>   3
                   NATIONAL-OILWELL RETIREMENT AND THRIFT PLAN

                       FINANCIAL STATEMENTS AND SCHEDULES

                                DECEMBER 31, 1999


                                TABLE OF CONTENTS


<TABLE>
<S>                                                                                                              <C>
Report of Independent Auditors....................................................................................4

Audited Financial Statements

         Statement of Net Assets Available for Benefits...........................................................5

         Statement of Changes in Net Assets Available for Benefits................................................6

         Notes to Financial Statements............................................................................7

Supplemental Schedule

         Schedule H, Line 4(i)--Schedule of Assets Held for Investment Purposes at
              End of Year........................................................................................12

Signature........................................................................................................13

Exhibit Index

         Exhibit 23.1--Consent of Independent Certified Public Accountants.......................................14
</TABLE>





<PAGE>   4

                         REPORT OF INDEPENDENT AUDITORS

The Benefit Plan Administrative Committee
National-Oilwell Retirement and Thrift Plan

We have audited the accompanying statements of net assets available for benefits
of National-Oilwell Retirement and Thrift Plan as of December 31, 1999 and 1998,
and the related statement of changes in net assets available for benefits for
the year ended December 31, 1999. These financial statements are the
responsibility of the Plan's management. Our responsibility is to express an
opinion on these financial statements based on our audits.

We conducted our audits in accordance with auditing standards generally accepted
in the United States. Those standards require that we plan and perform the audit
to obtain reasonable assurance about whether the financial statements are free
of material misstatement. An audit includes examining, on a test basis, evidence
supporting the amounts and disclosures in the financial statements. An audit
also includes assessing the accounting principles used and significant estimates
made by management, as well as evaluating the overall financial statement
presentation. We believe that our audits provide a reasonable basis for our
opinion.

In our opinion, the financial statements referred to above present fairly, in
all material respects, the net assets available for benefits of the Plan at
December 31, 1999 and 1998, and the changes in its net assets available for
benefits for the year ended December 31, 1999, in conformity with accounting
principles generally accepted in the United States.

Our audits were performed for the purpose of forming an opinion on the basic
financial statements taken as a whole. The accompanying supplemental schedule of
assets held for investment purposes at end of year as of December 31, 1999, is
presented for purposes of additional analysis and is not a required part of the
basic financial statements but is supplementary information required by the
Department of Labor's Rules and Regulations for Reporting and Disclosure under
the Employee Retirement Income Security Act of 1974. This supplemental schedule
is the responsibility of the Plan's management. The supplemental schedule has
been subjected to the auditing procedures applied in our audits of the basic
financial statements and, in our opinion, is fairly stated in all material
respects in relation to the basic financial statements taken as a whole.

                                                       /s/ Ernst & Young LLP  

May 25, 2000





                                      -4-

<PAGE>   5
                   NATIONAL-OILWELL RETIREMENT AND THRIFT PLAN

                 STATEMENTS OF NET ASSETS AVAILABLE FOR BENEFITS



<TABLE>
<CAPTION>
                                                                                DECEMBER 31
                                                                             1999         1998
                                                                         ------------------------
<S>                                                                      <C>          <C>     
ASSETS
Cash                                                                     $       908  $     5,531
Receivables:
   Employer contributions                                                     33,208    1,189,087
   Participant contributions                                                  48,134      317,109
   Accrued income                                                            290,433      636,009
                                                                         ------------------------
Total receivables                                                            371,775    2,142,205
Investments (Note 3)                                                      85,816,741   74,291,751
                                                                         ------------------------
Total assets                                                              86,189,424   76,439,487

LIABILITIES
Other                                                                            903        3,299
                                                                         ------------------------
Total liabilities                                                                903        3,299

                                                                         ========================
Net assets available for benefits                                        $86,188,521  $76,436,188
                                                                         ========================
</TABLE>






See accompanying notes.




                                      -5-

<PAGE>   6
                   NATIONAL-OILWELL RETIREMENT AND THRIFT PLAN

            STATEMENT OF CHANGES IN NET ASSETS AVAILABLE FOR BENEFITS


                          Year ended December 31, 1999


<TABLE>
<S>                                                                                     <C>     
Additions:
   Employer contributions                                                               $ 2,978,905
   Participant contributions                                                              3,499,545
   Participant rollovers                                                                    808,835
   Interest and dividends                                                                 1,108,381
   Net appreciation in fair value of investments (Note 3)                                14,693,007
                                                                                        -----------
Total additions                                                                          23,088,673
Deductions:
   Benefits paid to participants                                                         14,246,116
   Administrative expenses                                                                   82,287
                                                                                        -----------
Total deductions                                                                         14,328,403
Other changes in net assets:
   Net transfers to/from other qualified plans (Note 1)                                     992,063
                                                                                        -----------

Net increase                                                                              9,752,333

Net assets available for benefits at:
   Beginning of year                                                                     76,436,188
                                                                                        -----------
   End of year                                                                          $86,188,521
                                                                                        =========== 
</TABLE>

                                     





See accompanying notes.




                                      -6-

<PAGE>   7
                   NATIONAL-OILWELL RETIREMENT AND THRIFT PLAN

                          NOTES TO FINANCIAL STATEMENTS



                                DECEMBER 31, 1999


1. DESCRIPTION OF PLAN

The following description of the National-Oilwell Retirement and Thrift Plan
(the "Plan") is provided for general information only. Participants should refer
to the Summary Plan Description for a more complete description of the Plan's
provisions, a copy of which is available from National-Oilwell, L.P. (the
"Company").

GENERAL

The Plan was established effective April 1, 1987 for the benefit of the
employees of the Company. The Plan is a defined contribution plan covering
substantially all domestic employees who have completed at least six months of
service. The Plan is subject to the provisions of the Employee Retirement Income
Security Act of 1974 ("ERISA").

CONTRIBUTIONS

Participants may make both pretax and after-tax contributions to the Plan. The
maximum a participant can contribute (both pretax and after-tax combined) is 14%
of compensation, and the minimum is 1%. At its discretion, the Company may match
a portion of the participant's contribution. For 1999, the Company elected to
match 25% of the first 6% of each participant's contribution. The Company may
also make an additional discretionary matching contribution as of the end of
each Plan year, which is determined by the Company's financial performance
during the Plan year. For 1999, no additional discretionary match was made. The
Company made two other types of discretionary contributions to the Plan in 1999.
The first type of contribution, the Employer Retirement Contribution, is
allocated to participants' accounts based on their years of service. The second
discretionary contribution, the Employer Medical Savings Contribution ("MSA"),
was established to accumulate assets for payment of medical insurance premiums
after retirement, and is allocated to the participants' accounts based on years
of service. Each participant may direct the trustee to invest both the
participant's and the Company's contributions in one or more investment options.

NET TRANSFERS TO/FROM OTHER QUALIFIED PLANS

Effective December 1, 1999, the Dupre Supply Company, Inc. 401(k) Plan was
merged into the Plan as a result of the acquisition of the Dupre Supply Company,
Inc. in July 1999. Assets of approximately $1.896 million were transferred to
the Plan as a result of this merger. Certain participants from the Black Warrior
and Hiland divisions transferred balances to other qualified plans due to the
sale of those divisions.




                                      -7-

<PAGE>   8
                   NATIONAL-OILWELL RETIREMENT AND THRIFT PLAN

                          NOTES TO FINANCIAL STATEMENTS



1. DESCRIPTION OF PLAN (CONTINUED)

VESTING

Participants are immediately vested in participant and employer contributions
and the related earnings which have been credited to their accounts.

BENEFIT PAYMENTS

The Plan pays lump-sum benefits on retirement, disability, death, or termination
of employment. In-service withdrawals, subject to certain rules and
restrictions, may also be made from all but the retirement and MSA account
balances.

PARTICIPANT LOANS

The Plan includes a loan provision that permits participants to borrow up to the
lesser of $50,000 or 50% of the total value of their Plan assets. The loans are
payable in principal installments plus interest at prime plus 1% through payroll
deductions and are due in one-to five-year terms. The loans are considered
individually directed accounts which do not affect the assets of the other Plan
participants.

ADMINISTRATIVE EXPENSES

Certain administrative expenses are paid from the Plan's assets. All other Plan
expenses are paid by the Company.

PLAN TERMINATION

Although it has not expressed any intent to do so, the Company has the right
under the Plan to discontinue contributions at any time and to terminate the
Plan subject to the provisions of ERISA. Participants would be 100% vested in
their accounts in any event.

2. SUMMARY OF ACCOUNTING POLICIES

BASIS OF ACCOUNTING

The accompanying financial statements of the Plan have been prepared on the
accrual basis of accounting in accordance with accounting principles generally
accepted in the United States. Benefit payments to participants are recorded
upon distribution.




                                      -8-

<PAGE>   9
                   NATIONAL-OILWELL RETIREMENT AND THRIFT PLAN

                          NOTES TO FINANCIAL STATEMENTS


2. SUMMARY OF ACCOUNTING POLICIES (CONTINUED)

INVESTMENT VALUATION

Short-term investments are stated at cost, which approximates fair value.
Investments in corporate common stock and mutual funds are stated at fair value,
based on quotations obtained from national securities exchanges. Investments in
common collective funds are based on quoted market values as determined by
American Express based on the fair value of the underlying investments. The
investment contracts with insurance companies are fully benefit-responsive and,
therefore, are recorded at contract value which approximates fair value.
Contract value represents contributions made under the contract plus interest at
the contract rate less funds used to pay benefits and expenses. Participant
loans are stated at cost, which approximates fair value.

USE OF ESTIMATES

The preparation of financial statements in conformity with accounting principles
generally accepted in the United States requires management to make estimates
that affect the reported amounts in the financial statements and accompanying
notes and schedule. Actual results could differ from those estimates.

RECLASSIFICATION

Certain prior year amounts have been reclassified to conform to the current year
presentation.




                                      -9-

<PAGE>   10
                   NATIONAL-OILWELL RETIREMENT AND THRIFT PLAN

                          NOTES TO FINANCIAL STATEMENTS


3. INVESTMENTS

Individual investments that represent 5% or more of the Plan's net assets are as
follows:


<TABLE>
<CAPTION>
                                                                                    DECEMBER 31
                                                                                 1999         1998
                                                                             ------------------------
<S>                                                                          <C>          <C>
AET Equity Index II Fund                                                     $10,349,140  $ 8,333,986
AIM Balanced Fund                                                              6,171,870    6,335,471
AXP Growth Fund                                                               19,977,626   16,239,532
Janus Worldwide Fund                                                           9,350,806    5,574,061
AET Income II Fund                                                            21,702,131   14,442,585
Kentucky Commonwealth Life Insurance Company
   (GAC# ADA00804FR)                                                           4,522,272    4,315,492
Connecticut General Life Insurance Company
   (GIC #25198)                                                                        -    5,655,553

During 1999, the Plan's investments (including investments bought, sold, and
held during the year) appreciated in value as follows:

Common stock                                                                              $   943,610
Common collective funds                                                                     3,422,077
Mutual funds                                                                               10,327,320
                                                                                          -----------
                                                                                          $14,693,007
                                                                                          ===========     
</TABLE>


During 1999, Plan assets included a guaranteed investment contract with
Connecticut General Life Insurance Company ("CIGNA"), a group annuity contract
with Kentucky Commonwealth Life Insurance Company ("Kentucky Commonwealth"), and
a group annuity contract with New York Life Insurance Company ("New York Life").
Withdrawals of the funds' assets not in compliance with the contracts are
subject to certain penalties.




                                      -10-

<PAGE>   11
                   NATIONAL-OILWELL RETIREMENT AND THRIFT PLAN

                          NOTES TO FINANCIAL STATEMENTS


3. INVESTMENTS (CONTINUED)

The insurance companies maintain deposits in funds to which interest is added.
The rates in effect for the years ended December 31, 1999 and 1998 are as
follows:


<TABLE>
<CAPTION>
                                            YEAR ENDED DECEMBER 31                 MATURITY
                                            1999              1998                   DATE
                                     ------------------------------------- -------------------------
<S>                                         <C>               <C>          <C>
CIGNA                                       6.07%             6.07%        December 31, 1999
New York Life                               6.10%             6.10%        December 31, 1999
Kentucky Commonwealth                       6.41%             6.41%        January 2, 2001
</TABLE>


All rates are fixed through maturity.

4. INCOME TAX STATUS

The Plan has received a determination letter from the Internal Revenue Service
dated September 18, 1997, stating that the Plan is qualified under Section
401(a) of the Internal Revenue Code (the "IRC") and, therefore, the related
trust is exempt from taxation. Once qualified, the Plan is required to operate
in conformity with the IRC to maintain its qualification. The Plan has been
amended and restated since receiving the determination. However, the Benefit
Plan Administrative Committee believes the Plan continues to operate in
compliance with the applicable requirements of the IRC and, therefore, believes
that the Plan is qualified and the related trust is tax exempt.




                                      -11-

<PAGE>   12

                   NATIONAL-OILWELL RETIREMENT AND THRIFT PLAN

     SCHEDULE H, LINE 4(i) - SCHEDULE OF ASSETS HELD FOR INVESTMENT PURPOSES
                                 AT END OF YEAR


                                December 31, 1999

                             EIN: 76-0488987 PN: 001



<TABLE>
<CAPTION>
       IDENTITY OF ISSUE, BORROWER,                                                          CURRENT
         LESSOR, OR SIMILAR PARTY                      DESCRIPTION OF INVESTMENT              VALUE
-----------------------------------------------------------------------------------------------------------
<S>                                            <C>                                        <C>     
Kentucky Commonwealth Life                     Group Annuity Contract 
  Insurance Company                              #ADA00804FR                               $ 4,522,272

*American Express Trust Company                AET Money Market I Fund                         165,679

*National-Oilwell, Inc.                        242,618 shares of common stock                3,806,070

*American Express Trust Company                AET Equity Index II Fund                     10,349,140

*American Express Trust Company                AXP Bond Fund                                 1,681,881

*American Express Trust Company                AIM Balanced Fund                             6,171,870

*American Express Trust Company                Barron Asset Fund                             2,752,578

*American Express Trust Company                AXP Growth Fund                              19,977,626

*American Express Trust Company                AXP Blue Chip Advantage Fund                  3,596,919

*American Express Trust Company                Janus Worldwide Fund                          9,350,806

*American Express Trust Company                AET Income II Fund                           21,702,131

*Participant loans                             Various maturities and interest rates
                                                 ranging from 8.750% to 10.823%              1,739,769
                                                                                           -----------
                                                                                           $85,816,741
                                                                                           ===========
</TABLE>


*Party-in-interest




                                      -12-

<PAGE>   13
                                    SIGNATURE

         The Plan. Pursuant to the requirements of the Securities Exchange Act
of 1934, the trustees (or other persons who administer the employee benefit
plan) have duly caused this annual report to be signed on its behalf by the
undersigned hereunto duly authorized.

                                    NATIONAL-OILWELL RETIREMENT AND THRIFT PLAN




       June 14, 2000                    /s/ Daniel L. Molinaro
--------------------------------    --------------------------------------------
            Date                        Daniel L. Molinaro
                                        Treasurer for National-Oilwell, Inc. and
                                        National-Oilwell Benefits Plan 
                                        Administrative Committee Member





                                      -13-

<PAGE>   14
                                 EXHIBIT INDEX

EXHIBIT
NUMBER                            DESCRIPTION
------                            -----------

 23.1                   Consent of Independent Auditors

                                                  





<PAGE>   1
                                                                    EXHIBIT 23.1

                         CONSENT OF INDEPENDENT AUDITORS


We consent to the incorporation by reference in the Registration Statement (Form
S-8 No. 333-46459) pertaining to the National-Oilwell Retirement and Thrift Plan
of our report dated May 25, 2000, with respect to the financial statements and
schedule of the National-Oilwell Retirement and Thrift Plan included in this
Annual Report (Form 11-K) for the year ended December 31, 1999.

                                                           /s/ Ernst & Young LLP


June 8, 2000
Houston, Texas