replacement for its fighter aircraft, especially to counter the USSR's advanced Su-27  and  MiG-29 . Several companies submitted design proposals; the USAF selected proposals from  Northrop  and Lockheed . Northrop teamed with  McDonnell Douglas  to develop the YF-23, while Lockheed,  Boeing  and  General Dynamics developed the YF-22. The YF-23 was stealthier