The Pentagon’s fighter jet F-35 may not be fully operational until 2019 due to a newly discovered computer glitch. The $400 billion ultra-sophisticated jet, the most expensive in US history, was expected to enter service in 2015.

F-35 is the fifth generation combat aircraft which is designed in three variations for US Air Force, Navy and Marines to replace out of date aircraft. It was planned to join the Marines in 2015 and Air Force in 2016.

It is planned to replace the Air Force’s A-10 Warthog ground attack plane and Lockheed F-16 multirole fighter, the Navy instead of Boeing F/A-18 Hornet carrier-based fighter, and the Marines instead of Boeing AV-8B Harrier II jump-jet.

However, the most awaited plane’s main weapon will not be able to fire due to a computer glitch. The four-barreled rotary cannon for the Air Force version of the F-35 cannot function until new software is elaborated, despite jet scheduled to join the army this year.

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“There will be no gun until [the Joint Strike Fighter’s Block] 3F [software], there is no software to support it now or for the next four-ish years,” an Air Force official affiliated with the F-35 program told the Daily Beast. “Block 3F is slated for release in 2019, but who knows how much that will slip?”

This problem is especially acute as this version of the jet is planned for close air support (CAS) operations. While the F-35 is equipped with other armaments, it might be not enough.

“Lack of forward firing ordnance in a CAS supporting aircraft is a major handicap,” an experienced pilot commented for the Daily Beast. “CAS fights are more fluid than air interdiction, friendlies and targets move... Often times quickly. The ability to mark the target with rockets and attack the same target 10 seconds later is crucial.”

Equipped with a gun, Air Force’s F-35A version barely carries enough ammunition. Despite being able to shoot 3,300 rounds per minute, it will only be carrying 180 to 220 rounds.

The two other versions of F-35 - for the Navy and Marine Corps - have different configurations with external gun pods, however, they will not have a software for them either, the Daily Beast reported.

F-35 production has been facing delays and cost overruns due to numerous software problems and production defects. The jet which has already cost $400 billion dollars to US taxpayers since the program started in 2006. The costs doubled since the start of construction in 2011 making it the most expensive project in military history.

However, the Pentagon denied the aircraft will be delayed, the International Business Times reports as well as neither Lockheed nor the F-35 Joint Program Office responded to inquiries.

Last December, a problem with fuel was discovered. The engine of the aircraft can shut down when the fuel gets too hot to work as a coolant, although this information was disputed by the Pentagon.

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Last summer, the F-35 was supposed to be the star attraction at the Farnborough Air Show in Hampshire, but it did not appear after the entire fleet was grounded after a fire at a Florida airbase in June.

The Pentagon plans to buy 2,433 jets in three variations. Great Britain has also ordered 14 jets. However, the program is getting more and more severe criticism.

“To me, the more disturbing aspect of this delay is that it represents yet another clear indication that the program is in serious trouble,” an Air Force official told the Daily Beast. “F-35 maker “Lockheed Martin is clearly in a situation where they are scrambling to keep their collective noses above the waterline, and they are looking to push non-critical systems to the right in a moment of desperation.”

The F35 is designed and built by American Lockheed Martin Aeronautics Company which manufactures the widely spread military aircraft F-16, also known as the Fighting Falcon, successful all-weather multirole plane.