The modified contract increases the cap on retrofits needed on early production planes due to issues that arose in testing, which is still continuing, the U.S. Defense Department said in a daily listing of contract awards.
Lockheed said the change would enable the Defense Department to pay for certain changes they want incorporated into the new jets, up to the new cap. Lockheed is responsible for any additional costs beyond the agreed cap.
The Pentagon projects it will cost $397 billion to develop and build three variants of the plane for the U.S. military, a total of 2,447 planes. Eight partner countries also share in the development costs.
The U.S. Senate Armed Services Committee will hold a hearing on the new warplane, the Pentagon's costliest weapons program, on Tuesday afternoon.
Vice Admiral David Venlet, who runs the program for the U.S. government, is expected to provide an update on the program and several technical issues, including problems with the helmet worn by F-35 pilots, and the tailhook that helps stop the aircraft carrier variant of the plane.
About 3,650 Lockheed employees who work on the plane are in the third week of a strike at the company's Fort Worth, Texas facility and two sites where the U.S. military is carrying out test flights. The International Association of Machinists and Aerospace Workers wants to pressure Lockheed to drop its proposal to switch to a different pension system for new hires.
Lockheed officials said production and flight testing were continuing despite the strike. No new negotiating sessions have been agreed with the union.
(Reporting by Andrea Shalal-Esa; Editing by Tim Dobbyn)