( CNN ) -- The Pentagon 's most expensive weapons program ever , the F-35 warplane , is grounded again . Developed at a cost of nearly $ 400 billion so far and beset for years by cost overruns and delays , the so-called Joint Strike Fighter was put down temporarily this week following a runway fire at Eglin Air Force Base in Florida . No one was hurt . Pentagon spokesman Rear Adm. John Kirby said the cause of the June 23 incident remains under investigation , and the timing of when they might take off again is unclear . `` Additional inspections of F-35 engines have been ordered , and return to flight will be determined based on inspection results and analysis of engineering data , '' Kirby said . Engine maker Pratt & Whitney said in a statement that it was working with Air Force investigators to inspect all engines in the fleet . It declined further comment . Lead contractor Lockheed Martin also said it was working with investigators . It is producing variants of the plane for the U.S. Navy , Marines and Air Force . The Pentagon wants more than 2,400 ultimately , while hundreds more are expected over time to go to allies such as South Korea , Japan and Australia . More than 100 planes have been built so far , most for testing , but the program is still in its development and training phases . The military says the stealthy fighter will be `` the most affordable , lethal , supportable and survivable aircraft ever to be used '' by so many services worldwide . But its production has been controversial for its soaring cost history -- the price tag has nearly doubled from early estimates , to $ 135 million per unit as of last year , according to a U.S. Government Accountability Office report in March -- as well as its schedule , software and other setbacks . Test flights began in 2007 . While all models have been grounded in the past for various problems , the version for the Marines , the F-35B , has had more issues . Experts say that 's mainly due to its design for shorter takeoffs and vertical landings . Richard Aboulafia , a Teal Group analyst , said every component of the F-35 overall `` is pushing the frontiers of technology '' as engineers combine extraordinary engine power with a lighter weight design . He said the program has made gradual progress in recent years where glitches get resolved pretty quickly compared with 18 to 24 months ago , when setbacks seemed to come one right after another . `` It 's a tremendously complex project , '' he said , adding that cost issues remain a concern . The F-35 is due to make a much-anticipated appearance at the international air show at Farnborough airport in Hampshire , England , in coming weeks . Aboulafia said a no-show could indicate a more serious problem with the engine . U.S. sends B-2 stealth bombers to Europe . U.S. official : 'Dangerous ' Russian jet fly-by was 'straight out of a movie ' CNN 's Dana Ford and John Crawley contributed to this report .
Fire took place July 23 at Eglin Air Force Base . The Joint Strike Command is available for engine inspections . The F-35 program has been beset by delays and cost overruns over the years .