Air Force Secretary Frank Kendell said this week that five B-21 Raider stealth bombers are currently under production, three more than were previously known about. Photo courtesy of Northrop GrummanSept. 21 (UPI) -- An arms contractor is making "good progress" on the production of five B-21 Raider stealth bombers, U.S. Air Force Secretary Frank Kendall disclosed during a speech this week. B-21 Raider is the most advanced bomber to date and is being built for complex nuclear long-range missions. So far, the Air Force has only released renderings of the B-21. The Air Force wants the B-21 operational by the middle part of the decade and anticipates the aircraft's first flight in 2022.