The Hawker 400 is a small twin-engine corporate aircraft, designed by Raytheon's Beech Aircraft Company subsidiary, now part of Hawker Beechcraft.

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 United States Air Force 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. Another military variant is the Japan Air Self-Defense Force T-400 trainer which shares the same Type Certificate as the T-1A.

In 1993 Raytheon purchased the Hawker business jet product line from British Aerospace. The Beechjet 400 was eventually renamed the Hawker 400 to map it into the Hawker product line. The Hawker 400XP incorporates further aerodynamic, mechanical and interior improvements gleaned from the Hawker 800XP.

In October 2008, Hawker Beechcraft announced upgrades to the design, resulting in the new model designation Hawker 450XP. Upgrades include new, more fuel efficient engines (Pratt & Whitney PW535D models with 2,965 pounds of thrust apiece) Collins Pro Line 21 avionics and Collins Venue cabin management/entertainment system and a complete interior upgrade.