At the 1977 Paris air show Gates Learjet announced details of its new Learjet 50 series. This was intended to comprise the Learjet 54, Learjet 55 and Learjet 56 with longer and larger cabins. The first two were to seat up to 11 passengers and the last a maximum of eight; all were to have advanced wings incorporating NASA-developed winglets and this last feature gave rise to the name Longhorn. In fact, only the Longhorn 55 has been developed, construction of the first airframe starting in April 1978 after successful testing of anr Aerodynamic prototype of the wing on a Learjet 25. The first of two Learjet 55 prototypes was flown initially on 19 April,1979, and certification and delivery of the first production aircraft were recorded on 18 March 1981 and 30 April 1981 respectively.

The Model 55 followed the same overall configuration as earlier members of the Learjet family, and accommodated a crevv of two on a separate flight deck and up to 10 passengers in the main cabin. In September 1986 the company announced the Model 55B, offering a new interior, 'glass' cockpit, improved take-off performance and increased range, all of which increased the aircraft's operational flexibility. One year later, the Model 55C was revealed, fitted with rear underfuselage 'Delta Fins' giving improved directional stability at all speeds and reducing the landing speed, and thus removed the need for a stick puller/pusher. Later versions are the 55C/ER extended-range version fitted with an extra tank holding 1175kg of fuel in the tailcone, and the Model 55C/LR giving even-longer-range through the installation of a further 322-litre fuel cell behind the standard fuselage tank. Typical configuration of this version is two crew and seven passengers. Around 150 Model 55 variants had been built by the end of 1990 and the version is to be superseded by the Learjet 60 during 1992.