A fighter pilot sits in the cockpit while a crew member checks the exterior of a Lockheed Martin Corp. F-35A jet before a training flight in Hill Air Force Base, Utah, U.S., on Oct. 21. (George Frey/Bloomberg)

For the past several years, it had appeared that the Pentagon and Lockheed Martin had mended their once-strained relationship as they worked to fix the F-35 Joint Strike Fighter Program, which had gone wildly off course, marred by schedule delays and massive cost overruns.

But now the world’s largest military and its largest contractor are at a significant standoff over the most expensive weapons program in the history of the Defense Department.

Unable to reach an agreement on the price of the latest batch of fighter jets, the Pentagon used a rarely used provision Wednesday that allowed it to unilaterally execute the contract and set the price at its last offer.

In a statement, the Pentagon’s F-35 program office announced Wednesday that it had finalized its contract with Bethesda-based Lockheed for 57 of the next-

generation, stealthy fighter jets for $6.1 billion. That deal represented a 3.7 percent price decrease from the last batch of F-35s the Pentagon purchased, and Air Force Lt. Gen. Christopher Bogdan, the program’s executive officer, hailed it as “a fair and reasonable deal.”

But 12 minutes later, Lockheed issued a statement, saying the contract was not “mutually agreed upon,” and that the company was “obligated” to produce the aircraft under “previously agreed-to items.”

It said it was “disappointed” in the government’s action and that while it would “continue to execute” on the program it would also “evaluate our options and path forward.”

That means that it is deciding whether to appeal the contract, said an industry official with knowledge of the negotiations who was not authorized to speak publicly about them.

“The differences aren’t that far apart, but both sides felt they couldn’t move,” the official said.

Joe DellaVedova, a spokesman for the F-35 program office, said that after 14 months of negotiating, the Pentagon “believed further negotiations would not result in the government and Lockheed coming to an agreement on total price.” He said the unilateral action was in the “government’s best interest” and the price is “fair and reasonable.”

The F-35 program has, in years past, been one of the Pentagon’s most troubled procurements. The $400 billion program went billions over budget and fell years behind schedule. The aircraft was derided as the plane “that ate the Pentagon,” and critics such as Sen. John McCain (R-Ariz.) lambasted company and Pentagon officials for its failings.

Pentagon and industry officials said the inability to come to an agreement was not indicative of a problem in the relationship.

“We work together daily to deliver and support the F-35, and this will not change that,”

DellaVedova said.