With the first two P.1127(RAF) machines having flown by the end of 1966 development of the aircraft for full service use got into its stride. The remaining four aircraft flew during the first half of 1967, with the aircraft receiving the name earlier selected for the P.1154 - Harrier. A further development in the project had been launched in June 1966 when Hawker Siddeley had been authorised to undertake a twelve-month study of a two-seat trainer version of the aircraft. This was hoped to reduce the accident rate of the front line force, as well as providing additional combat capable aircraft for wartime use. The design that resulted, designated HS.1174, was for an aircraft with a 47-inch plug behind the cockpit, featuring a second seat stepped higher than the front seat. To balance the extra weight, a ballasted tail sting was added, with the fin being raised and moved aft to provide a counter to the increased area of the forward fuselage. The R & D cost of the trainer was estimated at around £10 million, with unit costs of around £1.15 million, compared to the £0.85 million of a single-seater.

With all the single-seat development aircraft having flown by mid-1967, test flying moved from handling trials onto the integration of the full weapons system. The nav-attack system adopted was based on the Ferranti FE541 inertial system, featuring a moving map display and weapon aiming computer, linked to a Specto head up display. This was one of the most advanced such systems in the world at the time, and it took considerable work to provide the required accuracy for navigation and attack - typically 1.5 nautical miles error after one hour's flight. A wide range of stores also needed to be cleared for delivery, including 1,000 lb. bombs, SNEB rocket pods and cluster bombs. Hawker Siddeley and the Aircraft and Armament Experimental Establishment, flying from Dunsfold and Boscombe Down, carried out these ordnance trials, as well as a range of field trials to ascertain the Harrier's ability to operate successfully from dispersed sites.

Harrier T.4, showing the raised cockpit, extended fin and 'tailsting' of the two-seater.	Detail of the nozzle actuation system on the Harrier.

It should be borne in mind that during the mid to late 1960s Britain still had considerable overseas military commitments. For the full buy of 110 aircraft the RAF planned to base one squadron in the UK, two in Germany and one each in the Middle East and Far East, in addition to a home-based Operational Conversion Unit. If only 60 aircraft were bought one German and the Middle East squadron would be dropped. In either instance, each squadron would have had a front-line strength of eleven single-seat and one two-seat Harrier. This meant that the aircraft had to be designed to operate in worldwide ambient temperatures, whether Arctic or tropical, as well as cope with the problems of ground erosion incurred when operating from sandy airstrips in the Middle East. It was only with the economic crisis of 1967 that led to the devaluation of the Pound, and the abandonment of most of Britain's overseas military commitments in the subsequent financial cutbacks, that the Harrier came to be seen by the RAF as an essentially NATO-based aircraft. Such a move eased some of the worries over logistics support that some of the aircraft's detractors had been pointing to - supplying dispersed sites using the excellent road infrastructure of Germany was far easier than supplying desert or jungle airstrips using dirt tracks. However, the economic crisis had once more directly threatened the Harrier programme with cancellation in order to save money. Denis Healey reluctantly allowed the programme to continue when he saw that the alternative Phantoms and Jaguars were now going to be both expensive and late into service compared to the Harrier, while his reluctant cancellation of the purchase of F-111K bombers eventually saved most of the sum demanded by the Treasury.

One of the factors that had triggered off the UK's economic crisis was the Six-Day War of 1967. The opening moves of the war, with the Israeli Air Force obliterating the air forces of its Arab opponents in a pre-emptive strike on their airfields, provided a vivid demonstration of the potential need for dispersed air power. The fact that the Harrier was already recognised as the only viable aircraft that could deliver such a capability helped it to survive the economic cuts in Britain as well as renewing interest in the aircraft from overseas. This interest included a potential order from Israel and possibly Finland, for 50 and ten aircraft respectively. However, it was from a totally unexpected direction that the most serious interest came - the United States Marine Corps.