After years of delays, technical problems and massive cost overruns, the most expensive warplane in history is finally about to make its international public flying debut.

The F-35 stealth fighter is set to roar through the skies over England next month, first at the Royal International Air Tattoo, then at the Farnborough International Airshow.

The Pentagon hopes the displays will convince program partners and prospective buyers that the plane's many problems may at last be behind it.

First envisioned in the early 1990s, the F-35 will eventually form the backbone of America's fighter fleet.

But the program has come with a ballooning price tag -- now at almost $400 billion -- for the nearly 2,500 planes that will be produced in the coming decades.

It was supposed to have flown at Farnborough -- one of the world's top air shows -- back in 2014, but the entire F-35 fleet was grounded when a plane's engine caught fire while training in the United States.

That incident was another in a string of blows for the F-35, which has suffered so many setbacks that at times is has seemed jinxed.

A Pentagon report in February said one version of the plane, the F-35A, remained dogged by problems, including software bugs and technical glitches. And issues with the eject system meant it could potentially kill pilots below a certain weight.

But industry watchers and Pentagon officials now say Lockheed Martin's Joint Strike Fighter, as it is also known, is progressing.

Lieutenant General Chris Bogdan, who is overseeing the program for the Pentagon, said most of the work developing the F-35 will be done by 2018.

The plane has actually already gone into service. A year ago, the US Marine Corps declared a batch of pared-down F-35Bs as operational, marking a major milestone.

The F-35 comes in three designs, with the F-35B capable of landing vertically and making short takeoffs. The F-35C is for use on aircraft carriers.

The Air Force is expected to declare its first F-35As, which conduct standard takeoffs and landings, as operational by the end of the year.

- 360-degree helmet -

Production levels are ramping up, and Bogdan said 53 aircraft should be delivered this year, compared to 45 in 2015. About 155 F-35s have been produced in all.

The eight other countries participating in the F-35 program are starting to get their planes, including the Netherlands, which this year took delivery of two F-35As in Leeuwarden.

"Technically, it has definitely seen the worst," said air industry expert Richard Aboulafia, vice president of analysis at the Teal Group.

Jerry Hendrix, the director of the strategies and assessments program at the Center for a New American Security, said the F-35 still has issues with its software and a sophisticated helmet that links to cameras and will give pilots a 360-degree view around the plane.

But "the F-35 has turned the corner from a technological standpoint and is coming along very rapidly," said Hendrix, initially an F-35 skeptic.

"The aircraft, with its engine, its wings, has come along and in fact is doing quite well right now."

Retired lieutenant general David Deptula, dean of the Mitchell Institute of Aerospace Power Studies, said pilots who have flown the F-35 acknowledge challenges but say the F-35 is potentially an "extraordinarily valuable addition to the nations that will be operating" it.

"The sensors provide the pilot with an enormous order of magnitude increase in knowledge or what pilots call situational awareness relative to any airplane that is out there today," he said.

- 'Additional sales' -

According to Lockheed Martin, the F-35 is supposedly far better at surveillance, aerial combat and ground attack than previous planes.

Experts say the F-35 has good commercial prospects, even though Canada -- one of the program's partner nations -- may dump it and seek an alternative fighter for its air force.

According to Bogdan, the cost of each F-35A is expected to drop to less than $85 million by 2019.

That's still double the cost of an F-16 fighter, Hendrix said, meaning only nations with large defense budgets can buy it.

"Given Russian and Chinese and Iranian investments in advance surface-to-air missiles as well as other (anti-plane defenses) this fifth-generation (fighter) is a necessary investment if you are going to be able to operate in this environment," Hendrix said.

"I suspect we will see additional sales."

The F-35 will be on non-flying display on the Dutch air base at Leeuwarden during an air show Friday and Saturday.

Five planes (three F-35Bs, two F-35As) are set to fly at the Royal International Air Tattoo (July 8-10), and three F-35Bs will fly at Farnborough the following week.

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LOCKHEED MARTIN