The F-35 Joint Strike Fighter is the Pentagon’s big plan for future warplanes — it’s slated to replace nearly all of the other tactical jets in the Air Force, Navy and Marine Corps. But getting there is going to be slow and expensive, as a new government report details.

The JSF program is a massively expensive undertaking. It has cost the government $400 billion to date, and is estimated to run more than $1 trillion to develop, buy and support nearly 2,500 aircraft through 2050.

A major problem, according to the Government Accountability Office report, is that the program is charging ahead with procurement while testing is still in progress. As Michael Sullivan, one of the report’s authors, told Congress, “the manufacturing processes are just never able to get stable because there's so much information coming in from testing and so many engineering changes that are going on.”

In a statement to Congress, the Pentagon officials in charge of the F-35 program said they "have put the program on sound footing for the future" but acknowledged that there was "no more money to put against contract overruns or problems."

The GAO report was notably stern. Here are some of its findings.

Only a portion of testing is finished:

Even though the design is not considered “mature,” production proceeds largely apace:

This leads to extra costs as already constructed planes have to be retrofitted:

The JSF’s software still needs work. Sullivan told Congress that “the development of the software that they need to make this aircraft fully combat capable is still as complex as anything on earth.”

“Supplier performance problems” and changes from testing have caused Lockheed Martin, the plane's manufacturer, to fall behind on production:

Because of safety and other engineering concerns, the Pentagon has reduced the number of planes it is ordering in the near future. But the program is still expected to cost more than $13 billion a year through 2035, and be an ongoing budget struggle for the Pentagon: