ey 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 accommodate the new engine and a armament of four 0.50-in
machine-guns, a different designated was given depending on which factory had built the plane. P-51B's were planes built in Inglewood and P-51C's were built at Dallas.
P-51B and P-51C's began to enter operational service with the USAAF in Britain as part of the 8th Air Force. The 13th October 1943 saw this new Mustang complete it's first long-range escort
mission when a force of bombers were sent to attack the U-boat yard at Kiel. The P-51 Mustang, with the benefit of external fuel tanks, would provide regular escort for bombers of the 8th
Air Force, which included the Boeing B-17 Flying Fortress and Consolidated B-24 Liberator, during their perilous daylight operations deep into the heart of Germany. During March 1944 the
Mustang took part in it's first mission to Berlin and started being used operationally by both the 10th and 15th Air Force in Burma and Italy respectively. The RAF also started to
receive their lend-lease P-51B/P-51C's, designated Mustang III, around the same time and No.19 Squadron based at Ford, Sussex were the first to receive this new type, which would eventually
be used by at least 21 RAF squadrons, many of which were part of the 2nd Tactical Air Force. The RAF modified the original cockpit canopy which opened sideways to a sliding hood design to
overcome the poor rear view of the original canopy design.
Development of the Mustang continued and the next production version was the P-51D, with a modified rear fuselage, six 0.50-in machine-guns and a bubble canopy as standard, this was to
become the most produced version of the P-51. Later versions of the P-51D had a small dorsal fin added and racks to accommodate 5-in rocket projectiles. A change in propeller lead to a
change in designation to P-51K, and both of these planes were designated Mustang IV and MK IVA respectively by the RAF.
During 1944 as part of experiments into lightweight construction the USAAF had ordered three XP-51F's and two XP-51G's as part of this process. A new redes