In 1977 Mitsubishi designed and built two prototypes of a twin-turbofan business aircraft designated the MU-300, the first flying on 29 August 1978. A cantilever low-wing monoplane with a pressurised fuselage and retractable tricycle landing gear, the MU-300 was powered by two JT15D-4 turbofan engines, pod-mounted one on each side of the rear fuselage. Standard accommodation was provided for a crew of two and seven passengers. At the end of the development programme the prototypes were dismantled and shipped to the USA, where they were reassembled by the company's US subsidiary Mitsubishi Aircraft International Inc. Redesignated the Diamond I, the two aircraft were used in the US certification programme, which was granted on 6 November 1981. Initial customer deliveries began in July
1982 and 62 were built.
An improved version, the Diamond IA, fitted with uprated JT15D-4D engines giving overall performance increases, an EFIS cockpit and with maximum take-off weight increased to 7361kg, was announced in
1983 and the first of 27 built, distin
guished by the extra port side window,
was delivered in 1984. With an MTOW
reduced to 7157kg, but with
extra fuel, and more powerful JT15D-5
engines, a further eight aircraft were pro
duced as the Diamond II. However, in
December 1985, Mitsubishi sold the
Diamond II design rights to Beech, together with components for 64 aircraft.
These were assembled at Wichita and
marketed as the Beechjet 400. Beech
then initiated full manufacture of the
type and by the beginning of 1991 had received orders for 113 slightly modified Model 400As In February 1990 the US Air Force chose the type as the airframe. element of the Tanker/Transport Training System under the designation Beech 400T T-1A Jayhawk. The USAF requirement is for 211 aircraft and the first of these was delivered in July 1991 and entered service in March 1992, training KC-135, C-5, KC-10 and C-17 crews.
