The F-111 Aardvark was an all-weather attack aircraft, capable of low-level penetration of enemy defences to deliver ordnance on the target. It was also the first production aircraft to feature variable-sweep wings. After a troubled development, the F-111 entered service with the US Air Force in 1967. It flew in Vietnam, Operation El Dorado Canyon, and the first Gulf War before its retirement in 1998.