The Pentagon wants $500 million more to finish the F-35 - the troubled jet that's already 50% over budget

Defense officials at the Pentagon say they need up to $500 million more to finish the development phase for the F-35, the troubled fifth-generation fighter that's already gone 50% over its original budget.

The F-35 program office requested the money last month to the Defense Acquisition Board, according to Bloomberg, which first reported the news Wednesday. The call for additional funds is pretty familiar at this point, since the program - known as the Joint Strike Fighter since it will be used by the Navy, Marines, and Air Force - has been plagued by lengthy delays and enormous cost overruns.

Its overall lifetime budget has ballooned to more than $1.5 trillion, making it the most expensive weapons system ever built by the US.

Sen. John McCain (R-Ariz.), the chairman of the Senate Armed Services Committee, has in the past called those cost overruns a "disgrace."

"It has been both a scandal and a tragedy with respect to cost, schedule and performance," he said in April.

Rising costs haven't been the only problem of note for the F-35. The jet has had plenty of incidents while being built, such as electrical problems, major issues with its software, and problems related to its advanced helmet system.

Just four months ago, the Pentagon's chief weapons tester wrote in a memo the F-35 program was "not on a path toward success but instead on a path toward failing to deliver."

Still, the Air Force and Marines have both declared the fighter "combat ready" and have begun integrating it their squadrons. The military has only taken delivery of about 180 of the aircraft from Lockheed Martin so far, though it plans to buy more than 2,400.

The fighter, which features stealth and advanced electronic attack and communications systems, is a project with roots going back to the late 1990s. Lockheed won the contract for the fighter in 2001.

"Strong national security is an expensive endeavor but the existing concerns with the F-35 make calls for even more money harder to green light," said Joe Kaspar, chief of staff for Rep. Duncan Hunter (R-Calif.), a member of the House Armed Services Committee.

"And the Pentagon never seems to be able to help its case on the F-35. Technical superiority is not cheap, but whether or not costs can be driven down is something Congress must look at it before throwing more money in the Pentagon's direction."