The aircraft was originally designed as the Mitsubishi MU-300 Diamond, an all-new, all-jet development to complement and slot above the Mitsubishi MU-2 and provide Mitsubishi Heavy Industries with their top-of-the-line corporate aircraft model (hence the name "Diamond"). Beechcraft bought the production rights and began manufacturing it as their own model, initially redesignated as the Beechjet 400. The Beechjet 400 was certified by the FAA in May 1985.
Raytheon/Beechcraft steadily developed their own improvements to the model, leading to the 400A in 1990. Improvements in the 400A include longer range, higher take-off weights and improved luxury appointments. An all-glass flight deck was also offered. Beechcraft also developed a version for the USAF known as the T-1A Jayhawk, used as a trainer for large aircraft crews (such as tankers and strategic transports). A total of 180 T-1A trainers were delivered between 1992-1997.