In this photo taken April 23, 2009, and released by U.S. Air Force, an F-35 Lightning II (also known as a Joint Strike Fighter) flies overhead for the first time at Eglin Air Force Base, Fla. (Senior Airman Julianne Showalter/U.S. Air Force via Associated Press)

Less than two months after declaring the controversial F-35 Joint Strike Fighter ready for combat, the Air Force on Friday announced that it was temporarily grounding 15 of the jets after it discovered that insulation was “peeling and crumbling” inside the fuel tanks.

The setback is the latest for the $400 billion system, the most expensive in the history of the Pentagon. The problem comes as the program, which for years faced billions of dollars in cost overruns and significant schedule delays, had begun to make strides. Last year, the Marine Corps had declared its variant ready for combat. In July, the Air Force gave a similar blessing to its variant.

Along with the manufacturer, Bethesda-based Lockheed Martin, Pentagon officials had declared that the long troubled program, derided as the “plane that ate the Pentagon,” had turned a corner. In testimony before Congress earlier this year, Lt. Gen. Christopher Bogdan, the program’s executive director, said the fighter is “at a pivot point” as production is about to ramp up. He said while 45 aircraft were manufactured in 2015, that number should grow to more than 100 in 2018 and up to 145 by 2020.

Even though the jet was declared combat-ready, Air Force officials warned this summer that it remains in development and that they could continue to find problems. The fact that the Pentagon committed to the aircraft before it was fully tested has been one of the chief criticisms of the program — a violation of procurement mantra: “Fly before you buy.”

“While nearing completion, the F-35 is still in development, and challenges are to be expected,” Air Force spokeswoman Ann Stefanek said in a statement. “The F-35 program has a proven track record of solving issues as they arise, and we’re confident we’ll continue to do so.”

In a statement, Lockheed Martin said that “the issue is confined to one supplier source and one batch of parts.” It emphasized that “this is not a technical or design issue; it is a supply chain manufacturing quality issue.”

Two of the grounded aircraft belong to Norway, the Air Force said. The insulation problem affects a total of 57 aircraft, the Air Force said, 42 of which are still in production. Lockheed Martin and Air Force officials are “developing procedures to resolve or mitigate the issue” before those aircraft become operational, the Air Force said.

In 2014, the Pentagon temporarily grounded the F-35 entire fleet after an engine in one of the jets caught fire as the jet was preparing to take off at Eglin Air Force Base in Florida, forcing the pilot to abort the flight. No one was injured.

In July, Gen. Herbert “Hawk” Carlisle, the head of the Air Force’s Air Combat Command, said in an interview he had faith in the aircraft and that he was “very, very confident it is going to continue to exceed our expectations.” But he warned that declaring the plane’s “initial combat capability is just the beginning. We still have work to do on the airplane, and it will continue to get better.”

The Air Force said Friday that crews discovered that insulation in avionics cooling lines inside the fuel tanks was deteriorating. It is unclear how long the aircraft would be grounded, how long the problem would take to fix or what the larger affect on the program would be.

Stefanek said the Air Force is “working with units to mitigate the impact on operations, training, and readiness.”

Lockheed said that “safety is always our first consideration” and that the company ” is committed to resolving this issue as quickly as possible to return jets to flying status.”