By Andrea Shalal

FARNBOROUGH, England (Reuters) - After 15 years of cost overruns and technical delays, the Lockheed Martin Corp <LMT.N> F-35 fighter jet is now knocking out "enemy" forces in combat exercises and surviving attacks in a way that even U.S. military officials say they did not expect.

The new stealthy warplane wowed crowds at two UK air shows over the past week, showing off its capabilities in what U.S. officials say is part of a larger drive to boost NATO's defenses and counter growing threats around the world.

But the real work is going on behind the scenes, including a series of combat exercises involving the more than 185 jets already delivered to the U.S. Marines Corp, Air Force and other countries such as Norway, Britain, the Netherlands and Australia; live fire weapons testing; and serious planning for how to use the jets in combat if needed.

There is some convincing to be done.

Critics say the F-35, at just under $100 million per plane, is far more costly than alternatives; that it will initially lack the ability to fire certain weapons and that it may be less capable in dogfights than older models.

But U.S. officials argue the plane's sophisticated fusion technology will let it spot enemy jets from such a distance that it never get into an actual dogfight, and that its cost will drop to around $85 million by 2019, stepping up competition with rivals such as Boeing Co F/A-18 and Eurofighter.

Pentagon acquisition chief Frank Kendall, who once described plans by his predecessors to buy three different F-35 models before they had been fully developed as "acquisition malpractice," said the program had clearly turned a corner.

The Marine Corps' first operational squadron of 10 F-35B jets, which can take off from shorter runways and land like a helicopter, will move permanently to Iwakuni, Japan, in less than six months, to be joined by six more jets in June 2017 when the USS Wasp amphibious ship arrives in the region.

"I can’t wait to get the airplane out to the Pacific," Lieutenant General Jon Davis, deputy commandant for aviation, told Reuters in an interview. "It’s tailor-made for that part of the world with its fifth generation capability and its expeditionary capabilities to land on a small ship or strip, and flow back and forth between those."

Davis says the F-35s are doing far better in combat exercises than expected, achieving so-called "kill ratios" of 24 to zero, and surviving every sort of simulated enemy attack.

"It is like watching a velociraptor going through. Everything in its path is killed," he told reporters.

"CONTINUOUS INVESTMENT"

Lockheed is building three variants of the F-35 for the U.S. military and 10 countries that plan to buy the jets: Britain, Australia, Norway, Denmark, the Netherlands, Italy, Turkey, Israel, Japan and South Korea. The U.S. portion of the program alone is slated to cost $379 billion, with a total of 3,000 jets to be fielded around the world in coming years.

Davis said the jets were ready for combat now if needed, a message echoed by Air Force General Hawk Carlisle, commander of Air Combat Command, which expects to declare an initial squadron of 12 F-35A jets ready for combat as early as next month.

Davis said the Marines were working on logistics for future missions and trying to ensure that small-deck carriers had enough bandwidth to take full advantage of the F-35's extensive data-gathering and sharing capabilities.

Air Force Lieutenant General Chris Bogdan, who runs the F-35 program for the Pentagon, said the jets' appearances at the UK air shows and other milestones were helping demonstrate to critics that the F-35 was more than "a paper airplane."

He said the world's largest weapons program was still working through some software glitches, but was "on the right track" to complete its development program in late 2017. He said it was likely take a real combat deployment or deterrence mission to dispel negative impressions after years of bad news.

"When we actually have the airplane out there ... and people realize how much of a leap in technology it is, then and only then will people start to realize that all the time and energy put into it is really worth it," Bogdan said in an interview.

He said the jets' performance in exercises like Red Flag showed that onboard radar sensors and electronic warfare equipment would give them a huge edge in future wars. While one-third to one-half of other aircraft would routinely be "killed" during tough exercises, no F-35s had been downed, he said.

The U.S. military is expected later this summer to review a series of upgrades planned for the jets over an eight-year period, that will cost just under $5 billion and add further weapons to the jet's capabilities - including the Small Diameter Bomb II built by Raytheon Co <RTN.N>.

Pentagon acquisition chief Kendall told reporters the work was necessary to ensure the F-35 kept its technological edge as potential foes raced to develop technology that could defeat it.

"We can't be complacent about that," he said. "We have to stay ahead of that, and that requires continuous investment."

(Reporting by Andrea Shalal; Editing by Mark Potter)