The Northrop/McDonnell Douglas YF-23 is an American single-seat, twin-engine, supersonic stealth fighter aircraft technology demonstrator designed for the United States Air Force (USAF). The design was a finalist in the USAF's Advanced Tactical Fighter (ATF) competition, battling the Lockheed YF-22 for a production contract. Two YF-23 prototype air vehicles were built.

In the 1980s, the USAF began looking for a replacement for its fighter aircraft to more effectively counter the USSR's advanced Sukhoi Su-27 and Mikoyan 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 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 developed into the Lockheed Martin F-22 Raptor, which first flew in 1997 and entered service in 2005. The U.S. Navy considered using the production version of the ATF as the basis for a replacement to the F-14, but these plans were later canceled. TFollowing the competition, both YF-23s were transferred to NASA's Dryden Flight Research Center at Edwards AFB, California, without their engines.[11][42] NASA planned to use one of the aircraft to study techniques for the calibration of predicted loads to measured flight results, but this did not take place.[42] Both YF-23 airframes remained in storage until mid-1996, when the aircraft were transferred to museums.[42][43] At the museums, the planes underwent restoration, during which it is believed that the aircraft's callsigns were switched. PAV-1 received "Gray Ghost" on the underside of its landing gear cover, while PAV-2 had "Spider" painted in the same location.