The restructured F-35 Lightning II Joint Strike Fighter program still has major problems, according to the newest audit by the Government Accountability Office.

“The program continues to experience cost growth and delays” and projected annual funding needs will average “more than $13 billion a year through 2035,” Michael J. Sullivan, the GAO director of acquisition and sourcing management, told the House Armed Services subcommittee on Tactical Air and Land Forces on Tuesday.

Overall, the most expensive weapons system now being built, the planned purchase of 2,457 aircraft over the next few decades is now expected to cost nearly $400 billion to develop and acquire and that doesn’t include its long-term operating expenses.

The program continues to be dogged by concurrency, which means developing structure, hardware and complicated software while testing at the same time the first production models from Lockheed-Martin.

For the third time in three years the Pentagon slowed down production rates scheduled through 2017. As a result only 365 will be procured by 2017, rather than the 1,591 that were planned in 2002. Since the program has yet to demonstrate “a stable design and manufacturing process capable of efficient production,” even with the planned reduction in near-term procurement, the Defense Department will have spent $69 billion for those 365 aircraft while flight testing continues, according to Sullivan’s report.

Up to now, the bill to retrofit the already-built aircraft is estimated at $373 million in addition to $672 million for the government’s share of cost overruns, says the GAO.

The propulsion system, being built by Pratt & Whitney, is also still under development although the company has delivered 42 production engines. Its estimated cost for development and demonstration has increased from $4.8 billion to $8.4 billion. Several contract due dates have been missed, but that has not mattered because of the production delays.

Software development for what has expected to be the most capable fighter ever is taking longer than expected because of its growth and complexity. The F-35 will have 9.5 million lines of code aboard the aircraft, three times that of its predecessor the F-22A. However, according to the GAO, the contractor has reported that “almost half of the on-board software has yet to compete integration and test -- typically the most challenging phase of software development.” Not surprising, only “four percent of mission systems requirements have been verified and significant learning and development remains before the program can demonstrate mature software and hardware” working together, the GAO found.

Fully integrated testing of the F-35 is tentatively scheduled for 2017 and a full production decision is now not expected until 2019, the GAO said.