the orders of Khrushchev to build the original two-engined helicopter, which for the first time in Soviet history would need purpose-built turbine engines, rather than those adapted from fixed wing aircraft (as in the Mil Mi-6 and the first prototype V-8) and an entirely new main rotor gear box that would be designed in-house for the first time. In May 1960, the order was given for Mikhail Mil to create his twin engine helicopter. The Sergei Isotov Design Bureau accepted the task of creating the engines. The second prototype (still equipped with the one turbine engine as the Isotov engines were still under development) flew in September 1961. Two months after the engines were completed by Isotov, the third prototype designated V-8A equipped with two 1,120 kW (1,500 shp) Isotov TV2 engines, made its first flight piloted by Nikolai Ilyushin on 2 August 1962, marking the first flight of any Soviet helicopter to fly with purpose built gas turbine engines. The aircraft completed its factory based testing in February 1963. The fourth prototype was designed as a VIP transport, with the rotor changed from four blades to five blades in 1963 to reduce vibration, the cockpit doors replaced by blister perspex slides and a sliding door added to the cabin. The fifth and final prototype was a mass production prototype for the passenger market. In November 1964, all joint testing had been completed and the Soviet government began mass production. Production started in the Kazan Production Plant, with the first aircraft completed by the end of 1965. The Soviet military originally showed little interest in the Mi-8 until the Bell UH-1's involvement in the Vietnam war became widely publicised as a great asset to the United States, allowing troops to move swiftly in and out of a battlefield and throughout the country. It was only then that the Soviet military rushed a troop-carrying variant of the Mil Mi-8 into production. By 1967, it had been introduced into the Soviet Air Force as the Mi-8. There are numerous variants, including the Mi-8T, which, in addition to carrying 24 troops, is armed with rockets and anti-tank guided missiles. The Mil Mi-17 export version is employed by around 20 countries; its equivalent in Russian service in the Mi-8M series. The only visible differences between the Mi-8 and Mi-17 are A) the position of the tail rotor (Mi-8 right side, Mi-17 left side), B)