The Harrier's ancestry began with the Hawker Siddeley P.1127. Design began in 1957 by Sir Sydney Camm, Ralph Hooper of Hawker Aviation and Stanley Hooker (later Sir Stanley) of the Bristol Engine Company. Rather than using rotors or a direct jet thrust the P.1127 had an innovative vectored thrust turbofan engine and the first vertical takeoff was on October 21^st 1960. Six prototypes were built in total.

The immediate development of the P.1127 was the Hawker Siddeley Kestrel FGA.1. The Kestrel's first flight was on March 7 1964. An order for 60 production aircraft was received from the RAF in mid-1966, and the first pre-production Harriers were flying by mid-1967. The Hawker Siddeley Harrier GR1 was the first production model derived from the Kestrel, it first flew on December 28^th 1967, and entered service with the RAF on April 1^st 1969. The Harrier GR3 featured improved sensors (such as a laser tracker in the lengthened nose), countermeasures and a further upgraded Pegasus Mk 103.

Development of a successor to the first Harrier began as a cooperative effort between McDonnell Douglas (U.S.) and Hawker Siddeley (U.K.). Cost overruns eventually led Hawker to withdraw from the project, but work continued due to U.S. interest in the aircraft. Britain re-entered development in the late 1970s, producing their own version of the Harrier II based on the U.S. design. The first BAE built development aircraft the mark GR5 flew for the first time on April 30^th 1985 and the aircraft entered service in July 1987.

There have been a number of upgrades first being the mark GR7 with the uprated Rolls-Royce Pegasus 105 engine. The Harrier GR9 is an avionics and weapons upgrade of the standard GR7. Alongside the GR9 upgrade, some aircraft are being fitted with more powerful engines to enable them to perform better in extremely hot climates and are designated as GR9A. Under a contract awarded to BAE Systems in 2004, new digital weapons that will be integrated onto the GR9 will include the advanced GPS and laser-guided Paveway IV bomb and infrared and television variants of the Maverick missile to achieve high precision ground attack capabilities. The aircraft is also expected to carry the advanced Brimstone fire and forget anti-armour missile. Longer term plans for the aircraft include equipping secure communications, a ground proximity warning system and for training the Rangeless Airborne Instrumentation and Debriefing System (RAIDS). The programme also includes an upgrade to the two-seater T10 training aircraft to T12, the equivalent of the GR9 standard.

The Harrier GR9 & T10/T12 two-seater aircraft are operated at three airfields, RAF Cottesmore with 1 and 4 Squadron, 20 (R) Squadron at RAF Wittering and 41 (R) Squadron at RAF Coningsby.