The Gloster E.28/39, (also referred to as the Gloster Whittle, Gloster Pioneer, or Gloster G.40) was the first British jet-engined aircraft and first flew in 1941. It was the fourth jet to fly, after the German Heinkel He 178 (1939), the Italian Caproni Campini N.1 motorjet (1940), and the German Heinkel He 280 (1941).

The E.28/39 was the product of a specification which had been issued by the Air Ministry for a suitable aircraft to test the novel jet propulsion designs that Frank Whittle had been developing during the 1930s. Gloster and the company's chief designer, George Carter, worked with Whittle to develop an otherwise conventional aircraft fitted with a Power Jets W.1 turbojet engine. Flying for the first time on 15 May 1941, a pair of E.28/39 aircraft were produced for the flight test programme. Following initial satisfactory reports, these aircraft continued to be flown to test increasingly refined engine designs and new aerodynamic features. Despite the loss of the second prototype, due to improper maintenance causing a critical aileron failure, the E.28/39 was considered to be a success.

The E.28/39 contributed valuable initial experience with the new type of propulsion and led to the development of the Gloster Meteor, the first operational jet fighter to enter service with the Allies. The first prototype continued test flying until 1944, after which it was withdrawn from service; in 1946, it was transferred to the Science Museum in London, where it has been on static display ever since; full-scale replicas have been created.