US officials said Thursday that the military and civilian chiefs of the Air Force are resigning. Defense officials who spoke on condition of anonymity said Defense Secretary Robert Gates asked Air Force Chief of Staff Gen. Michael Moseley and Air Force Secretary Michael Wynne to step down. A public announcement was expected later. There was no immediate word on who would be nominated to replace Moseley and Wynne. The Air Force has endured a number of embarrassing setbacks over the past year. In August, for instance, a B-52 bomber was mistakenly armed with six nuclear-tipped cruise missiles and flown across the country. The pilot and crew were unaware they had nuclear arms aboard. The error was considered so grave that President George W. Bush was quickly informed.