The Lockheed F-117 Nighthawk is a retired American single-seat, twin-engine stealth surface–attack aircraft developed by Lockheed's secretive Skunk Works division and operated by the United States Air Force (USAF). It was the first operational aircraft to be designed around stealth technology. The F-117 was based on the Have Blue technology demonstrator. The Nighthawk's maiden flight took place in 1981 at Groom Lake, Nevada, and the aircraft achieved initial operating capability status in 1983. The aircraft was shrouded in secrecy until it was revealed to the public in 1988. Of the 64 F-117s built, 59 were production versions, with the other five being prototypes. The F-117 was widely publicized for its role in the Persian Gulf War of 1991. Although it was commonly referred to as the "Stealth Fighter", it was strictly an attack aircraft. F-117s took part in the conflict in Yugoslavia, where one was shot down and another damaged by surface-to-air missiles (SAM) in 1999. The U.S. Air Force retired the F-117 in April 2008, primarily due to the fielding of the F-22 Raptor. Despite the type's official retirement, a portion of the fleet has been kept in airworthy condition, and Nighthawks have been observed flying since 2009.[4]