The Gloster E.28/39, (also referred to as the "Gloster Whittle", "Gloster Pioneer", or "Gloster G.40") was the first jet engined aircraft to fly in the United Kingdom. Developed to test the new Whittle jet engine in flight, the test results would influence the development of an operational fighter, the Gloster Meteor.
In September 1939, the Air Ministry issued a specification to Gloster for an aircraft to test one of Frank Whittle's turbojet designs in flight. Working closely with Whittle, Gloster's chief designer George Carter laid out a small low-wing aircraft of conventional configuration. The jet intake was in the nose, and the tail-fin and elevators were mounted above the jet-pipe. A contract for two prototypes was signed by the Air Ministry on 3 February 1940 and the first of these was completed by April 1941. Building started in Hucclecote near Gloucester, but was later moved to Regent Motors in Cheltenham High St (now the Regent Arcade), considerd a location safer from bombing.
The E.28/39 name comes from the aircraft having been built to the 28th "Experimental" specification issued by the Air Ministry in 1939.
Source - Internet.