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 re-designated 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.