The Northrop/McDonnell Douglas YF-23 was a single-seat, twin-engined prototype fighter aircraft designed for the United States Air Force (USAF). It was a finalist in the USAF's Advanced Tactical Fighter (ATF) competition, battling the Lockheed YF-22 for a production contract. Two YF-23s were built, nicknamed "Black Widow II" and "Gray Ghost".
In the 1980s, the USAF began looking for a replacement for its fighter aircraft, especially to counter the 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, using some existing components from the F-15 Eagle and F/A-18 Hornet, was stealthier and faster but less agile than its competitor. After a four-year development and evaluation process, the YF-22 was announced the winner in 1991 and entered production as the Lockheed Martin F-22 Raptor. The U.S. Navy considered using one of the ATF aircraft to replace the F-14, but later canceled these plans. The two YF-23s were donated to museums and are now exhibits.
Source: Wikipedia, "Northrop YF-23", available under the CC-BY-SA License.