Lockheed Martin Corp’s’s new F-35 fighter jet has completed over a third of its planned flight tests, but it still faces problems with the helmet needed to fly the plane, software development and weapons integration, according to a report by the Pentagon’s chief weapons tester.

The 18-page report, sent to Congress on Friday, included a detailed account of those issues and others, which it said underscored the “lack of maturity” of the $396 billion weapons program, the Pentagon’s most expensive ever.

The program exceeded the number of flight tests and specific system tests planned for 2012 but lagged in some areas due to unresolved problems and newly discovered issues, the report said. It said Lockheed did not accomplish all the tests planned for 2012, but boosted the year’s total of specific tests by bringing forward some evaluations planned in later years.

The program has already completed over 20,000 tests, but has 39,579 more such tests.

The report highlighted the continued growing pains of the ambitious Lockheed fighter program, which began in 2001 and has been restructured three times in recent years to slow down production and allow more progress on the development program.

Lockheed said the F-35 program continued to show progress on flight test, software development and other aspects of the reworked plan, and was demonstrating exceptional stability -- more than any other legacy aircraft development program.

“It’s more important to look at the overall plan rather than year by year totals,” Lockheed spokesman Michael Rein said in an emailed statement. “While we remain diligent to ensure deferred test objectives are ultimately completed, the aggregate plan remains on track.”

Lockheed is building three different models of the F-35 fighter jet for the U.S. military and eight countries that helped pay for its development: Britain, Canada, Italy, Turkey, Denmark, the Netherlands, Australia and Norway.

The Pentagon plans to buy 2,443 of the warplanes in coming decades, although many analysts believe U.S. budget constraints and deficits will eventually reduce that overall number.