The two YF-23 prototype aircraft fly over the Mojave Desert prior to arrival at NASA's Dryden Flight Center, Edwards, California.The YF-23 prototype is one of two aircraft designed and built by the contractor team of Northrop and McDonnell Douglas as part of the demonstration and evaluation phase of the U.S. Air Force's Advanced Tactical Fighter (ATF) selection program.Both YF-23 prototypes were transferred from Norththrop to Dryden. There were no engines in the two aircraft, and NASA had no plans to fly the YF-23's in any research program.NASA had planned to use one of the two aircraft to extensively study strain gage loads calibration techniques, while the other would remain in storage at Dryden. However, both aircraft remained in storage until the summer of 1996 when the aircraft were transferred to museums. The YF-23A Prototype Air Vehicle 2 (PAV-2), serial # 87-0801, is on display at the Western Museum of Flight in Hawthorne, California, on long term loan from NASA. YF-23A PAV-1 (87-0800) is currently at the Air Force Flight Test Center Museum at Edwards Air Force Base, California. Both YF-23 prototypes were designed and built by the contractor team of Northrop and McDonnell Douglas as part of the demonstration and evaluation phase of the U.S. Air Force's Advanced Tactical Fighter selection program, which concluded in 1990. The YF-23 was unofficially known as the ''Black Widow II,'' a reference to the WWII-era Northrop P-61 Black Widow night fighter.