Navistar International Corp (NAV): MHR Fund Management Acquires More Shares

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SECURITIES AND EXCHANGE
COMMISSION

WASHINGTON, DC 20549

SCHEDULE 13D

[Rule 13d-101]

INFORMATION TO BE INCLUDED IN STATEMENTS FILED

PURSUANT
TO § 240.13d-1(a) AND AMENDMENTS

THERETO FILED PURSUANT TO § 240.13d-2(a)

Amendment No. 17

Navistar
International Corporation

(Name of Issuer)

Common Stock, par value $0.10 per share

(Title of Class of Securities)

63934E108

(CUSIP Number)

Janet Yeung

MHR Fund Management LLC

1345 Avenue of the Americas, 42^nd Floor

New York, New York 10105

(212) 262-0005

(Name, Address and Telephone Number of Person Authorized to Receive Notices of Communication)

September 5, 2016

(Date of Event Which Requires Filing of This Statement)

If the filing
person has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because of §§240.13d-1(e), 240.13d-1(f) or
240.13d-1(g), check the following box. ¨

Note: Schedules filed in paper format shall include a signed original and five copies of the Schedule, including all exhibits. See §240.13d-7 for
other parties to whom copies are to be sent.

The information required on the remainder of this cover page shall not be deemed to be filed for the purpose of
Section 18 of the Securities Exchange Act of 1934 (the Act) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

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Explanatory Note

This statement on Schedule 13D (this Statement) amends and supplements, as Amendment No. 17, the Schedule 13D filed on June 15, 2012 (the Initial
Schedule 13D), which was amended on June 25, 2012 by Amendment No. 1 to the Initial Schedule 13D (Amendment No. 1), on July 10, 2012 by Amendment No. 2 to the Initial Schedule 13D
(Amendment No. 2), on October 9, 2012 by Amendment No. 3 to the Initial Schedule 13D (Amendment No. 3), on October 26, 2012 by Amendment No. 4 to the Initial Schedule 13D
(Amendment No. 4), on December 12, 2012 by Amendment No. 5 to the Initial Schedule 13D (Amendment No. 5), on July 15, 2013 by Amendment No. 6 to the Initial Schedule 13D
(Amendment No. 6), on July 17, 2013 by Amendment No. 7 to the Initial Schedule 13D (Amendment No. 7), on March 11, 2014 by Amendment No. 8 to the Initial Schedule 13D
(Amendment No. 8), on April 11, 2014 by Amendment No. 9 to the Initial Schedule 13D (Amendment No. 9), on June 24, 2015 by Amendment No. 10 to the Initial Schedule 13D
(Amendment No. 10), on December 18, 2014 by Amendment No. 11 to the Initial Schedule 13D (Amendment No. 11), on January 30, 2015 by Amendment No. 12 to the Initial
Schedule 13D (Amendment No. 12), on June 11, 2015 by Amendment No. 13 to the Initial Schedule 13D (Amendment No. 13), on July 27, 2015 by Amendment No. 14 to the Initial Schedule 13D
(Amendment No. 14), on September 4, 2015 by Amendment No. 15 to the Schedule 13D (Amendment No. 15) and on December 18, 2015 by Amendment No. 16 to the Initial Schedule 13D
(Amendment No. 16 and, together with Amendment No. 1 through Amendment No. 15 and the Initial Schedule 13D, the Schedule 13D), and relates to shares of common stock, par value $0.10 per
share (the Common Stock), of Navistar International Corporation (the Issuer). Except as otherwise provided, capitalized terms used in this Statement but not defined herein shall have the respective meanings
given to such terms in Amendment No. 16.

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Item 4. Purpose of Transaction

Item 4 is hereby amended by adding the following:

The information set forth in Item 6 below is incorporated into this Item 4 by reference.

Item 5. Interests in Securities of the Issuer.

Item 5(a)(viii)
and Item 5(b)(viii) are hereby amended by deleting such items in their entirety and replacing them with the following:

(a)(viii) Dr. Rachesky
may be deemed to be the beneficial owner of 16,264,104 shares of Common Stock (approximately 19.9% of the total number of shares of Common Stock outstanding, calculated in accordance with Rule 13d3(d)(1)(i) under the Act). This number consists
of (A) all of the shares of Common Stock otherwise described in this Item 5 by virtue of Dr. Racheskys position as the managing member of each of Advisors, Institutional Advisors III and MHR Holdings, (B) 15,532 shares of Common Stock held
directly, (C) 20,000 shares of Common Stock that can be obtained upon the exercise of certain stock options, and (D) 3,572 shares of Common Stock that can be obtained upon the settlement of phantom stock units.

(b)(viii) Dr. Rachesky may be deemed to have (x) the sole power to direct the disposition of 16,264,104 shares of Common Stock which may be deemed to be
beneficially owned by Dr. Rachesky as described above, and (y) the sole power to direct the voting of 16,264,104 shares of Common Stock which may be deemed to be beneficially owned by Dr. Rachesky as described above.

Item 6. Contracts, Arrangements, Understandings or Relationships With Respect to the Securities of the Issuer.

Item 6 is hereby amended by adding the following:

On September 5, 2016, at the Issuers request, the Reporting Persons, together with certain of their affiliates, and the Issuer entered into Amendment No. 2 (Amendment No. 2) to
the Settlement Agreement, dated October 5, 2012, as amended and restated by Amendment No. 1 to the Settlement Agreement, dated July 14, 2013. Pursuant to Amendment No. 2, the Issuer is permitted to increase the maximum size of its board of directors
from ten to twelve directors.

The foregoing description of Amendment No. 2 does not purport to be complete and is qualified in its entirety
by reference to the full text of Amendment No. 2, a copy of which is filed as Exhibit 1 to this Statement and is incorporated by reference herein.

Item 7. Material to be Filed as Exhibits

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SIGNATURES

After reasonable inquiry and to the best of our knowledge and belief, the undersigned certify that the information set forth in this
Statement is true, complete and correct.

Dated: September 6, 2016

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