e, an order had been placed before the
prototype had even flown and so more of the aircraft soon followed.
Designated Mustang I by the RAF, initial evaluation showed that at low-level the plane was fast and extremely manoeuvrable and was much better than any other US fighter then available.
At higher altitude the performance of the plane suffered as its Allison engine power output fell rapidly as it climbed, meaning that despite the Mustang's promise it wasn't suitable for
a combat role in Europe.
With its eight machine-guns comprising four 0.50-in and four 0.303-in and it's superb performance at low-level it was within this role that the Mustang would serve with No.2 Squadron of Army
Co-operation Command during April 1942, equipped with obliquely-mounted cameras and on on the 27th July they flew their first operational sortie. Just three months later however and the
Mustang showed its potential for long range escort duties when in October 1942 on an attack on the Dortmund-Ems canal it became the first RAF single-engined fighter to cross the German
border from its base in Britain, however the performance of the aircraft at higher altitude still needed to be improved, but an order for another 300 aircraft was placed.
One of the conditions to allow North American to supply the RAF with it's NA-73 design was that two aircraft were given to the United States Army Air Corps (USAAC) for evaluation under the
designation XP-51. Before these two aircraft were supplied to the USAAC the US Army had already ordered 150 more aircraft to be sent to Britain under the lend-lease programme.
These planes featured self-sealing tanks and instead of eight machine-guns featured four 20-mm cannons and were designated P-51 by the US and Mustang IA by the British.
A number of these planes went to the United States Army Air Force (USAAF) designated F-6A and were used for tactical reconnaissance and were fitted with two K-24 cameras. And two examples
initially designated XP-78 but later XP-51B were each tested with different engines and they proved very successful and the RAF's findings of superb performance at low-level was confirmed.
However the US Army was committed to both the Lockheed P-38 Lightning and Republic P-47 Thunderbolt, nevertheless 500 P-51's modified to have dive breaks and bomb racks to provide close
support to ground troops and designated A-36 Apache were ordered. First flown during September 1942 and powered by a Allison V-17110-87 engine with six 0.50-in machine-guns, they were the
first Mustang variant to go into operational service with the USAAF. Equipping two groups based in the Middle East during 1943 they also performed support operations during the invasions
of Sicily and Italy. As well as ordering the A-36 an order for just over 310 P-51A's had been placed by the US Army with armament of four 0.50-in machine-guns and racks for either 1,000lb
of bombs or two of either 75 or 150 US gallon external fuel tanks, this was designated Mustang II by the RAF and powered by a 1,200-hp Allison V-1710-81 engine.
After showing it's ability for potential long-range escort duties into Germany, but lacking high-altitude performance, during 1942 the decision was taken to install Rolls-Royce Merlin 61 and
65 engines in four airframes of the Mustang MK I. Testing showed a much improved performance and North American were informed of the results and a 1,430-hp US-built Packard Merlin V-1650-3
engine was installed into two P-51s, these would be designated XP-78/XP-51B. Tested during September 1942 and achieving a maximum speed of 441 mph and having a better rate of climb than
the P-38 Lighting, and confirming the British findings, the USAAF impressed with the performance ordered the Merlin powered Mustang in large numbers.
Due to North American not having the production capacity at their factory in Inglewood, California another planet was built at Dallas, Texas. Despite being the same plane, which featured
the following improvements over the P-51 and P-51A, of a strengthened fuselage, improved ailerons and a number of small changes to accom