The Northrop YF-17 Cobra was part of the "Lightweight-Fighter" program of the US Air Force. The program was started because of the high costs of the F-15 Eagle, so that a cheaper aircraft was needed to be produced in greater numbers, as well as interest for a new aircraft for the export market. The YF-17 is a further development of the F-5 and it failed against the YF-16, which was later developed into the Lockheed Martin F-16 Fighting Falcon.
After losing in the Lightweight-Fighter program the YF-17 was redesigned by McDonnell Douglas for the "Naval-Fighter-Attack-Experimental“ program, while the single engine F-16 Fighting Falcon was not powerful enough and unuseable for carrier service. The YF-17 was evolved into the F/A-18 Hornet.