The Northrop YF-23, unofficially nicknamed the "Black Widow II", was a stealth fighter prototype developed by Northrop and McDonnell Douglas for the U.S. Air Force's Advanced Tactical Fighter (ATF) program in the late 1980s and early 1990s. Designed to replace the F-15 and counter advanced Soviet threats, the YF-23 competed against the Lockheed YF-22 (which became the F-22 Raptor) but lost the competition in 1991 primarily due to the YF-22's superior agility, despite the YF-23 being faster and more stealthy.