The General Accounting Office calls the F-35 Joint Strike Fighter “the most expensive and ambitious” weapons project ever undertaken, with acquisition costs of about $400 billion and operating costs of more than $1 trillion over the life of the program.

And the costs to develop the stealthy jet are still rising.

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In a just-released report, the GAO says the Department of Defense has begun planning and funding “the development of new capabilities” that will cost taxpayers more than $3.9 billion between now and the end of 2022.

The brief report to the House and Senate Armed Services Committee is primarily about process – whether the so-called “follow-on modernization” should be designated as a major defense acquisition program (MDAP) of its own or whether it should be managed under the existing F-35 program, as the Pentagon prefers.

But the implication in the language of the report is that spending on upgrades to the F-35 will go on indefinitely.

The GAO refers to the $3.9 billion for research, development and testing of new capabilities as funds for “the first modernization phase,” suggesting that others will follow.

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Although the GAO pegs acquisition costs at about $400 billion, in a report submitted to Congress last month, the Pentagon itself projected that the price tag for producing 2,456 of the stealth jets will rise to $406.5 billion, according to Bloomberg.

And in April the GAO said it expected “cascading delays” would add $1.7 billion to the F-35 program costs, with $1.3 billion in 2018 alone.

That report also recommended that DOD finish developmental testing of the F-35 before moving ahead with significant new investments.