It is among the U.S. Air Force’s most finite resources, but the B-2 Spirit stealth bomber is still growing in capabilities, and it’s an aircraft America is happy to allow an ally to use. A long-standing exchange program that has seen USAF personnel trade places with those from the Royal Air Force was extended to the B-2 in 2004. The offer to open up the secretive B-2 mission to a key ally was made by then-President George W. Bush in an e-mail to then-Prime Minister Tony Blair. Among the achievements of the first British pilot, Sqn. Ldr. David Arthurton, was...