The Boeing Vertol CH-46 Sea Knight is a medium-lift tandem rotor transport helicopter, used by the United States Marine Corps (USMC) to provide all-weather, day-or-night assault transport of combat troops, supplies and equipment. Assault Support is its primary function, and the movement of supplies and equipment is secondary. Additional tasks include combat support, search and rescue, support for forward refueling and rearming points, CASEVAC and Tactical Recovery of Aircraft and Personnel (TRAP). The commercial version is the BV 107-II, commonly referred to as simply the "Vertol".
Piasecki was a pioneering developer of tandem-rotor helicopters, with the most famous previous helicopter being the H-21 "Flying Banana". Piasecki's former company Vertol began work on a new tandem rotor helicopter designated the Vertol Model 107 or V-107 in 1956. The V-107 prototype had two Lycoming T53 turboshaft engines, producing 877 shp (640 kW) each. The first flight of the V-107 took place on 22 April 1958. The V-107 was then put through a flight demonstration tour in the US and overseas. In June 1958, the US Army awarded a contract to Vertol for ten production aircraft designated "YHC-1A".
A door gunner manning a pintle-mounted .50-caliber machine gun aboard a USMC CH-46.
The order was later decreased to three, so the Army could divert funds to the V-114, also a turbine powered tandem, but larger than the V-107. The Army's three YHC-1As were powered by GE-T-58 engines. The YHC-1As first flew in August 1959, and were followed by an improved commercial/export model, the 107-II. During 1960, the U.S. Marine Corps (USMC) evolved a requirement for a twin-turbine troop/cargo assault helicopter to replace the piston engine types then in use. Following a design competition, Boeing Vertol was selected to build its model 107M as the HRB-1, early in 1961. Boeing had acquired Vertol in 1960 and renamed the group Boeing Vertol