of the helicopter has been exported to nations across the world; the U.S. Army and the Royal Air Force (see Boeing Chinook (UK variants)) have been its two largest users. The civilian version of the Chinook is the Boeing Vertol 234. It has been used by civil operators not only for passenger and cargo transport, but also for aerial firefighting and to support logging, construction, and oil extraction industries. Design and development Background During late 1956, the United States Department of the Army announced its intention to replace the Sikorsky CH-37 Mojave, which was powered by piston engines, with a new, gas turbine-powered helicopter. Turbine engines were also a key design feature of the smaller UH-1 "Huey" utility helicopter. Following a design competition, in September 1958, a joint Army–Air Force source selection board recommended that the Army procure the Vertol-built medium transport helicopter. However, funding for full-scale development was not then available, and the Army vacillated on its design requirements. Some officials in Army Aviation thought that the new helicopter should be operated as a light tactical transport aimed at taking over the missions of the old piston-engined Piasecki H-21 and Sikorsky H-34 helicopters, and be consequently capable of carrying about 15 troops (one squad). Another faction in Army Aviation thought that the new helicopter should be much larger, enabling it to airlift large artillery pieces and possess enough internal space to carry the new MGM-31 "Pershing" missile system. During 1957, Vertol commenced work upon a new tandem-rotor helicopter, designated as the Vertol Model 107 or V-107. During June 1958, the U.S. Army awarded a contract to Vertol for the acquisition of a small number of the rotorcraft, giving it the YHC-1A designation. As ordered, the YHC-1A possessed the capacity to carry a maximum of 20 troops. Three underwent testing by the Army for deriving engineering and operational data. However, the YHC-1A was considered by many figures within the Army users to be too heavy for the assault role, while too light for the more general transport role. Accordingly, a decision was made to procure a heavier transport helicopter, and at the same time, upgrade the UH-1 "Huey" to serve as the needed tactical troop transport. The YHC-1A would be improved and adopted by the Marines as the CH-46 Sea Knight in 1962. As a result, the Army issued a new order to Vertol for an enlarged derivative of the