In March 1954, Hawker decided to embark on the development of a new fighter aircraft in response to the release of Operational Requirement 323 (OR.323) by the Air Ministry.

The proposed design was reworked the following year when, in February 1955, the Ministry issued OR.329, which, amongst other requirements, called for a large 40-inch (100 cm) sweep-scanning radar unit, AI.18, to be installed.

[5] In May 1957, Camm produced a rough draft of a twin-engined version of the aircraft, designated as the P.1125, which was to have been powered by a pair of Rolls-Royce RB.133 engines.

[5] While OR.329 was cancelled, it was recognised that the design could be reworked to meet OR.339, which was by then the only-remaining long term manned aircraft requirement left to contend for.

During October 1957, shortly after the receipt of OR.339 that month, it was determined that an enlarged version of the P.1125 proposal was the most likely candidate to meet the requirements, thus, in January 1958, it was submitted as the P.1129, powered by a pair of Rolls-Royce RB.141 engines.

[5] On 25 June 1959, the Defence Research Policy Committee approved funds for OR.339, at which point Hawker Siddeley's management directed that a re-submission be made.

[6] However, the Hawker Siddeley board decided to slow development on the project, being unwilling to self-finance its final completion along with its maiden flight, pending upon the outcome of OR.339.

[11] Conventional construction was employed throughout the majority of the P.1121; using a thick lightweight alloy skin placed on top of tightly spaced frames and stringers, while steel was used for some highly stressed components.

[11] The fuselage was split into three major sections; the front fuselage contained the radome, cockpit, radio, and equipment bay, the centre section housed the engine, air intake, belly-mounted air brake, nosewheel, 2-inch (51 mm) rocket launching fixtures, and a total of five nylon bag fuel tanks.

[11] The spars were attached to steel booms at the inboard end, which were in turn fixed onto the four main frames of the centre fuselage section.

[12] A fully integrated power control system with autopilot functionality and Doppler navigation was to assist in performing low-altitude bombing runs using both conventional and nuclear armaments.