WASHINGTON — The Pentagon has awarded Lockheed Martin a $1.3 billion contract for advance work on 13 F-35s added to an 11th batch of joint strike fighters during budget negotiations last year, according to the program office.

The contract includes six Marine Corps F-35Bs, three Air Force F-35As and four Navy F-35Cs, according to an announcement Monday. Work will be completed in December 2019.

Because the 13 aircraft were not expected when contract negotiations began, no money was initially included for long lead work, according to the joint program office. The Pentagon asked for two F-35s over the planned buy in its fiscal 2016 budget submission, and Congress added 11 more aircraft in the omnibus spending bill — a total of 13 extra jets.

The money will go toward adding the 13 F-35s into the production schedule of the other aircraft appropriated in FY16, which are included in the 10th low-rate initial production lot, according to the JPO.

Lockheed is still waiting to finalize negotiations for the ninth and 10th batches of F-35s, valued at about $16 billion total. JPO Chief Lt. Gen. Christopher Bogdan told reporters recently that he expected to finalize the ninth low-rate initial production contract with Lockheed by the end of March, with the 10th following a few months later. But negotiations on both are still ongoing.