The Learjet 23 is an American six to eight seat (two crew and four to six passengers) twin-engine, high speed business jet. Manufactured by Learjet, the LJ23 was revolutionary in the aviation world. With this jet a completely new market for fast and efficient business aircraft was opened. The LJ23 is considered as a model for a whole set of similar airplanes which remain in production.

The first flight of the LJ31 took place on 11 May 1987. The Learjet 31A variant was introduced in October 1990. This version featured increased cruising speed, a digital avionics system with EFIS supplied by AlliedSignal (today Honeywell) and an instrument panel layout change. The nose gear wheel is steered by a Steer by Wire system. The windshield could be heated electrically.

The Learjet 40 is derived from the Learjet 45, but with a shorter fuselage (by 24.5 inches/60 cm), and is powered by two Honeywell TFE731-20AR engines. These are known as the "AR" engines.