Throughout its service life, the aircraft received several upgrades, which were typically focused upon its engines, radar and weapons, including support for the De Havilland Firestreak air-to-air missile.

Several variants were proposed, including fighter bomber and aerial reconnaissance-oriented models, as well as the supersonic-capable Gloster thin-wing Javelin; however, these were not pursued.

In the aftermath of the Second World War, Britain identified a threat posed by the jet-powered strategic bomber and atomic weaponry and thus placed a great emphasis on developing aerial supremacy through continuing to advance its fighter technology, even following the end of the conflict.

It would also have to reach a maximum speed of 525 kn (973 km/h, 605 mph) at this height, be able to perform rapid ascents and attain an altitude of 45,000 feet within ten minutes of engine ignition.

[4][page needed] The later-issued P.234 and P.238 of early 1947 had adopted many of the features that would be distinctive of the Javelin, including the large delta wing and tailplane, and were based on the Rolls-Royce AJ65 engine (better known as Avon).

[7] The RAF requirements were subject to some changes, mainly in regards to radar equipment and armaments; Gloster also initiated some changes as further research was conducted into the aerodynamic properties of the new swept and delta wings, as well as use of the new Armstrong Siddeley Sapphire turbojet engine.

[8] On 13 April 1949, the Ministry of Supply issued instructions to two aircraft manufacturers, Gloster and de Havilland, to each construct four airworthy prototypes of their competing designs to meet the requirement, as well as one airframe each for structural testing.

These prototype aircraft were the Gloster GA.5, designed by Richard Walker, and the de Havilland DH.110, the latter of which held the advantage of also being under consideration for the Royal Navy.

[11][12] Bill Waterton, Gloster's Chief Test Pilot, would later describe the Javelin as being "as easy to fly as an Anson",[13] although also expressing concern over its inadequate power controls.

The Javelin had experienced a "deep stall"; the wing acting like an airbrake had killed forward motion and at the same time degraded the airflow over the elevators, leaving them useless.

[18] On 4 July 1954, a prototype Javelin accidentally achieved supersonic speed during a test flight, the pilot having been distracted by an oxygen supply failure.

[20] While some delays were incurred, the Javelin's status as a "super priority" for production helped to minimize the time involved in producing each aircraft.

Production was assisted by a large order placed by the United States Air Force, purchasing aircraft for the RAF as part of the Mutual Defense Aid Program at a price of £36.8 million.

[20] On 21 October 1954, a pilot attached to Gloster from RAE Farnborough was killed while flying Javelin XA546 having entered what appeared to be an intentional spin.

[27][28] Changes from the prototypes included alterations to the rear fuselage and a central "pen nib" fairing extending beyond the engine nozzles, to eliminate buffeting of the rudder by the jet exhaust and increased sweepback of the wing's leading edge to improve high-speed handling.

[32] The turnaround time between sorties was significantly shorter than with the preceding Gloster Meteor, due to improved ground accessibility and engine ignition sequence.

In spite of the aircraft's unorthodox aerodynamic features, the Javelin had a fairly conventional structure and materials, being mainly composed of an aluminium alloy, with some use of steel edging.

[46] The final incarnation of the thin-wing Gloster (P.376) just before cancellation was a large aircraft carrying two Red Dean all-aspect missiles as a possible contender for Operational Requirement F.155.