Since its inception in 1957, the P.1127/Kestrel/Harrier series and their Pegasus engine had received strong support from the United States. Although the US Marine Corps had not taken part in the Tripartite trials, the arrival of the six XV-6As in the United States in 1966 allowed some Marine pilots to fly the aircraft. Although the US Air Force and Navy saw the XV-6A trials simply as academic exercises, the Marines were impressed by the aircraft's qualities of simplicity and flexibility. Despite this, the lack of real military 'punch' offered by the XV-6A meant that the Marine's interest did not progress any further for the moment. It was by the development of the Harrier that Marine interest was rekindled, and after a promotional film of the Harrier undergoing flight trials reached the Marines in 1968 they quickly decided that they needed some hands-on experience of the aircraft. It was to this end that two Marine pilots, Colonel Tom Miller and Colonel Bud Baker, walked into the Hawker Siddeley chalet at the 1968 Farnborough Air Show and announced that they wished to fly the Harrier.

Sidewinder armed AV-8A with refuelling probe.	Dramatic front view of an AV-8A, showing the size of the intakes.

Hawker Siddeley were naturally both surprised and delighted at this turn of events, and within a few weeks the two Marine pilots were on their way back to the United States to report on the trial flights that they had been given at Dunsfold. Their only difficulty appeared to be containing their enthusiasm - the trials had confirmed all the Marines' hopes about the Harrier. By early 1969 a US Navy team was at Dunsfold carrying out a more detailed assessment of the aircraft. Although it was through the Navy that the Marines purchased their aircraft, making the second evaluation necessary, it was also vital to gain support in Congress for the purchase of this foreign aircraft. It was Congress that controlled the military budget, and it was a long-standing view 'on the Hill' that the United States should not rely on foreign contractors for their weapons. However, the Marine Corps was, politically, the best connected of the Services, with the widespread view being held in Congress that 'what the Marines want, the Marines get'. After ensuring that the planned purchase of 114 Harriers would only take place if the majority of aircraft were manufactured under licence in the United States, the Marines were allowed to order an initial twelve Harriers for delivery direct from Hawker Siddeley, the aircraft initially being allocated the service designation of AV-6B, soon changed to AV-8A.

Throughout 1969 Hawker Siddeley talked to several American companies in order to satisfy the licence production requirement. With the 1963 agreement with Northrop having lapsed, as well as the 1962 agreement with Republic aviation to co-operate on any US purchase of the P.1154, it was eventually decided that McDonnell Douglas would be the best company for the task. The two companies were already linked through the UK purchase of the F-4 Phantom II, Hawker Siddeley being the nominated UK design and support authority for these aircraft. This link with McDonnell Douglas was to have a profound long-term influence on the Harrier, but in the medium-term it was found to be impractical to produce the AV-8A in the US - the system of funding yearly batches made the setting up of an American production line uneconomic for the numbers envisaged.