Today's Beechjet 400A is much changed and improved aircraft from the days when it started out life as the Mitsubishi MU300 Diamond I. Originally designed and built in Nagoya, Japan, the Diamond prototype made its first flight back in August 1978. The second prototype aircraft, assembled in San Angelo Texas, flew August 10, 1979 and was awarded FAA certification May 18, 1980. The aircraft continued to be improved through 1985, first as the Diamond IA and then as the Pratt & Whitney JT15D-5 powered Diamond II. In early 1986 Beech Aircraft acquired manufacturing and distribution rights to the Diamond II, made some operator-suggested updates to the aircraft and renamed it the Beechjet 400.
The Beechjet 400 was in production from 1986 to 1989, after which Beech moved the entire manufacturing line from Japan to Wichita, Kansas. In 1990 the Beechjet 400A debuted as an aircraft produced entirely in the US. Improvements at the time included certification to 45,000 ft, a double-club cabin arrangement, an aft lav and a Collins EFIS flightdeck. The most recent evolution of the aircraft, beginning in 1995 with s/n RK110, features a cabin redesign with a center club seating arrangement, improved sound insulation, better environmental systems and a number of flightdeck upgrades.