The Learjet 55 was Lear’s entry into the medium sized business jet field. In designing the 55, Learjet utilizes the earlier Longhorn 28/29 wing with winglets and marries it to a larger fuselage. The step-down aisle in the cabin has 5 ft 9 inches of headroom. Power is supplied by two Honeywell TFE 731 engines that produce 3,700 pounds of thrust each. The Learjet 55B introduced a digital flight deck, modified wings, and an improved interior. The 55C comes equipped with Delta Fins that improve performance and handling. The first production aircraft flew on August 11, 1980.