FORT WORTH — The Pentagon has finalized terms on more than $4 billion in modifications to contracts with Lockheed Martin Aeronautics covering the production of 40 F-35 jet fighters.In late July, Lockheed reached an agreement with the Defense Department on contract terms for what’s known as production lots 6 and 7 of the F-35 program, which lowers the cost per plane to the government. The contracts awarded Friday finalize terms of that agreement.One contract, for $3.4 billion, covers 19 planes to be built for the Air Force, six for the Marine Corps, four for the Navy and six for England, Italy and Norway. A second modification, worth $743 million, covers five planes for Australia and Italy.More than half the work on the contracts will be done in west Fort Worth, where Lockheed Martin has thousands of workers dedicated to the F-35. Other work will be done at Lockheed facilities in El Segundo, Calif.; Orlando, Fla.; and elsewhere. Production is expected to be completed in 2016. Lorraine Martin, F-35 general manager for Lockheed Martin, said the contracts represent “a significant milestone for the F-35 program and its path to enhanced affordability.”“With each successive production lot, unit costs have declined,” she said. “That’s a trend we look forward to continuing as this program moves toward full-rate production and operational maturity.”Lockheed Martin has been working to stabilize costs on the huge F-35 program, which has a history of technical problems and cost overruns. Pentagon leaders said in June that they were cautiously optimistic that F-35 production could be ramped up, and Lockheed said 2,400 manufacturing jobs in Fort Worth could be added over several years. Lockheed Martin employs more than 14,000 in west Fort Worth.