Cobra, was derived from the UH-1, and retained a high degree of commonality. The Bell 204 and 205 are Iroquois versions developed for the civilian market. In response to demands from some customers, a twin-engined model, the UH-1N Twin Huey, was also developed during the late 1960s; a four-bladed derivative, the Bell UH-1Y Venom, was also developed during the early twenty-first century. In US Army service, the Iroquois was gradually phased out following the introduction of the Sikorsky UH-60 Black Hawk and the Eurocopter UH-72 Lakota, although hundreds were still in use more than 50 years following the type's introduction. In excess of 16,000 Iroquois have been built since 1960. Development In 1952, the U.S. Army identified a requirement for a new helicopter to serve as medical evacuation (MEDEVAC), instrument trainer, and general utility aircraft. The Army determined that current helicopters were too large, underpowered, or too complex to maintain easily. During November 1953, revised military requirements were submitted to the Department of the Army. Twenty companies submitted designs in their bid for the contract, including Bell Helicopter with the Model 204 and Kaman Aircraft with a turbine-powered version of the H-43. On 23 February 1955, the Army announced its decision, selecting Bell to build three copies of the Model 204 for evaluation with the designation XH-40. Model 204 Powered by a prototype Lycoming YT53-L-1 (LTC1B-1) engine producing 700 shp (520 kW), the XH-40 first flew on 20 October 1956, at Fort Worth, Texas, with Bell's chief test pilot, Floyd Carlson, at the controls. Even prior to the first flight, the Army had placed an order for six YH-40 service test helicopters. During 1957, a further two prototypes were completed. In March 1960, the Army awarded Bell a production contract for 100 aircraft, which was designated "HU-1A" and officially named Iroquois after the Native American nations. The helicopter quickly developed a nickname derived from its HU-1 designation, which came to be pronounced as "Huey". The reference became so popular that Bell began casting the name on the helicopter's anti-torque pedals. The official U.S. Army name was almost never used in practice. Even after September 1962, at which point the designation for all models was changed to UH-1 under a unified Department of Defense (DOD) designation system, yet the nickname persisted. While glowing in praise for the helicopter's advances over piston-engined helicopters, the Army reports from the service tests of the