The F-35 Lightning II during the test phase. This is the variant that would be used aboard aircraft carriers. (US Navy/US NAVY)

It is supposed to be the headliner of the show next month, when the most expensive fighter jet ever built makes its international debut in England. There will be plenty of other planes at the Farnborough Air Show, but none will get as much attention as the F-35 Lightning II, the futuristic, sleek and stealthy jet that proponents say is unparalleled in the history of human flight.

But this week, an F-35A, the Air Force’s variant of the much-
heralded plane, caught fire, forcing the pilot to abort takeoff. It led to the suspension of scores of training flights across the country as investigators scramble to figure out what went wrong.

The fire in the rear of the plane, which was doused by emergency crews at Eglin Air Force Base in Florida on Monday and resulted in no injuries, was the second problem in the past two months. In June, an oil leak discovered by a pilot in flight led to another suspension of flights.

While Pentagon officials say the F-35 is still expected to make its appearance overseas next month, and some of the planes returned to flight status Friday, the problems have sowed even more doubt about a program that was for years over budget and behind schedule. And it comes as the manufacturer, Bethesda-based Lockheed Martin, is trying to sell the jet to international clients in an effort to keep production costs down as U.S. defense spending tightens.

“Potential customers are mindful that this program has had a lot of teething problems,” said Richard Aboulafia, an aerospace analyst at the Virginia-based Teal Group. “If this is cleared up quickly, it won’t be a problem. But if it persists through Farnborough, that’s going to hurt.”

Lockheed has had some recent success selling the plane overseas, particularly to South Korea, Israel, Japan and Australia. And Farnborough, a large gathering of aerospace companies and military buyers from all over the globe, was seen as a prime opportunity to showcase the F-35 to more foreign governments.

But critics and competitors are likely to pounce on the recent mishaps, Aboulafia said. In recent months, Boeing has taken aim at the F-35, saying that its EA-18G Airborne Electric Attack Aircraft, known as the Growler, is better at providing stealth.

This month, Sen. John McCain (R-Ariz.), a longtime critic of the F-35, said on the Senate floor that “this is clearly a program that has had and continues to have major issues.”

Kyra Hawn, a spokeswoman for the F-35 joint program office, said Friday that the Air Force and Navy continued to keep their version of the aircraft on the ground.

The fire might have caused damage to the aircraft’s stealth coating, she said, making it the first possible Class A mishap — incidents that cause $1 million in damage or more.

Marine Corps Capt. Richard Ulsh said in a statement Friday afternoon that the F-35B, the version that is scheduled to appear at Farnborough, would resume flights Friday. He declined to elaborate on why the Marine Corps felt its version was safe to fly while others remained on the ground.

“We are continuing with our plans to deploy to the U.K. next month,” he said.

In a sign that military officials are proceeding with plans to show the plane overseas, four jets arrived at Patuxent River Naval Air Station on Friday from Yuma, Ariz., in anticipation of the trip to Britain, officials said.

Before heading to Farnborough, the F-35 is expected to fly at the Royal International Air Tattoo in Britain.