Known at the F-35 Lightning II, the combat aircraft is the most expensive weapons system under development and promoted as the future of the U.S. air fleet.
The combat planes were to make their debut in 2013 but now plans may have to wait for as long as 2015, Air Force Secretary Michael Donley said this week.
"We are going to have a slip," Donley was quoted saying by The Washington Post.
It was not immediately clear whether delays in the F-35 program would affect the U.S. Marines and U.S. Navy, which anticipate begin using the new planes by 2012 and 2015, respectively.
The project involves nine other nations, including Britain, Italy, Turkey, Canada, Norway and Australia.
"Troubled," as he put by the delays, U.S. Defense Secretary Robert Gates fired last month the general leading the program, saying he would also withhold $614 million from the contractor, Lockheed Martin.
Now the Obama administration wants Congress to approve $11.4 billion for the program, on top of the $8.4 billion purchase price for the 43 planes charted for initial production.
"We want to hold the contractors' feet to the fire," Donley was quoted saying. "We want to incentivize them to make good on the promises they made earlier and deliver on schedule."
Concerns over the cost overruns and delays in the project have precipitated a probe by a U.S. Senate committee expected to examine the matter next week.
Defense Department officials have been scrambling to allay skittish senators of problems in the much-vaunted program, but to no avail.
"There's a lot of questions that need to be answered," said Sen. John McCain, R-Ariz., adding that senators had not been thoroughly informed about the program and its problems.
Lockheed Martin has remained tight-lipped saying in a statement that it was "fully committed to the F-35 program" and that it was working toward stabilizing "cost and affordability -- and to fielding the aircraft on time."
Industry experts, however, are wary.
The secretary of defense reluctantly supports this program because he has no other alternative," Mackenzie Eaglen, an analyst with the Heritage Foundation, told the Post.
"The (F-35) is like a sweater … You pull any thread, like pushing back on full-rate production, and things can fall apart very quickly," he said.
"A delayed start will have a ripple effect of steadily increasing the average age of the Air Force's inventory."
The Pentagon has said that it wants to buy 2,500 of the combat aircraft over the next 25 years. It has set the cost at about $300 billion.
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