YF-23 is an American single-seat, twin-engine, stealth fighter technology demonstrator prototype designed for the United States Air Force (USAF). The design team, with Northrop as the prime contractor, was a finalist in the USAF's Advanced Tactical Fighter (ATF) demonstration/validation competition, battling the YF-22 team for full-scale development and production. Nicknamed "Black Widow II", two YF-23 prototypes were built.