The US Air Force plans to spend $2.8 billion to keep old combat aircraft in the air because of major delays with the new F-35 fighter jet program, top officials said Friday.

With the production schedule of the F-35 jet repeatedly postponed due to technical problems, the Pentagon will upgrade 350 aging F-16 fighters to fill the gap in the fleet, Air Force leaders told reporters.

“The issue with respect to F-35 is that obviously the planes are not delivering as quickly as originally anticipated,” said General Norton Schwartz, the Air Force chief of staff.

As a result, the Air Force needed “to posture the legacy force to make sure that we retain the capabilities we need until the F-35 delivers in numbers,” he said.

The F-35 Joint Strike Fighter, manufactured by Lockheed Martin, is touted as the backbone of America’s future air fleet but an ambitious production timeline has unraveled due to technical headaches that emerged in initial flight tests.

Officials discussed extending the service life of the F-16s as they unveiled details of the Air Force’s proposed budget for fiscal year 2013.

Despite production delays for the F-35, Air Force Secretary Michael Donley insisted the government remained fully committed to the program and to purchasing a total of 2,443 of the aircraft as planned.

“This is a must-do for our armed forces. It’s the future of the fighter force, not only for the Air Force, Navy and Marine Corps, but also about 12 other international partners as well,” he said.

At an estimated $385 billion, the F-35 is the Pentagon’s most expensive weapons program. The military had hoped to have 423 of the F-35 fighters built between 2013 and 2017 but has had to slash the number down to 244.