The Beech Aircraft Corporation acquired in December 1985 the Diamond II executive jet from Mitsubishi, and it is now marketed worldwide (except for Japan) as the Model 400 Beechjet. Initial Beech production centred on the completion of partially-built Mitsubishi airframes, but by June 1989 all production was being handled by Beech's Salina and Wichita plants.

The first Beech-assembled aircraft was rolled out on 19 May 1986 and deliveries began in June. By the beginning of 1989 46 Beechjets had been delivered. Beech is also providing support for previous Mitsubishi products, including the MU-2 and earlier Diamond versions.

In 1989, Beech announced the Model 400A Beechjet, featuring increases in payload and ceiling, greater cabin volume, a rear lavatory and improved soundproofing. A Collins Pro Line 4 EFIS is fitted as standard. Deliveries of this version began in early 1990.

To compete for the major USAF competition for a new Tanker-Transport Training System, Beech teamed with McDonnell Douglas and FlightSafety International to offer the Beechjet. The team was successful, with an eventual order for 211 aircraft likely. Expected to enter service in 1992, the aircraft will be known as the T-1A Jayhawk.