The U.S. Air Force is making progress on a new long-range bomber, Air Force Secretary Michael Donley said at a trade conference in Florida last week. “There’s a competition,” Donley said, according to DoDBuzz.

“The program is underway, the requirements, the cost parameters have been set by the Secretary of Defense and we’re executing in that direction…we’ve identified the target delivery for the mid ‘20s.”

The Air Force has been working on a bomber since 2006, according to most reports, with the aim of first complementing then replacing the existing fleet of 150 B-52, B-1 and B-2 bombers, the oldest of which date from the early 1960s. Then-U.S. Secretary of Defense Robert Gates suspended the bomber development in 2009, citing out-of-control cost and technical ambition.

But Gates’ successor, Leon Panetta, revived the bomber work as part of the Pentagon’s “strategic pivot” towards the Asia-Pacific region, whose long distances pose huge problems for military planners – and play to the strengths of heavy bombers. The so-called “Long-Range Strike Bomber” is meant to be stealthy, capable of carrying nuclear weapons and “optionally-manned,” meaning it can be remotely flown without a pilot on board.

Boeing, Northrop Grumman and Lockheed are all competing for the Long-Range Strike Bomber contract. The fleet of up to 100 new bombers is expected to cost at least $55 billion over the next 20 years.