The F-35 Joint Strike Fighter, the supposed backbone of the Pentagon’s future air arsenal, could need additional years of work and billions of dollars in unplanned fixes, the Air Force and the Government Accountability Office revealed on Tuesday. Congressional testimony by Air Force Secretary Michael Donley, plus a new report by the GAO, heaped bad news on a program that was already years late, tens of billions of dollars over its original budget and vexed by mismanagement, safety woes and rigged test results.