 The YF-16's first supersonic flight was accomplished on 5 February 1974, and the second YF-16 prototype first flew on 9 May 1974.This was followed by the first flights of Northrop's YF-17 prototypes on 9 June and 21 August 1974, respectively.During the flyoff, the YF-16s completed 330 sorties for a total of 417 flight hours; the YF-17s flew 288 sorties, covering 345 hours.