The first production Mustang Is appeared in April 1941. Tests revealed an aircraft with a top speed of 382 mph at 13,000 feet. When it entered service in 1942 it had the best low level performance of any RAF fighter.
The Mustang I carried a mixed armament of eight machine guns. Two .50 inch machine guns were mounted under the engine cowling, firing through the propellers. Each wing contained two .30 inch machine guns and one .50 inch machine gun, arranged with the bigger gun between and below the two .30s. Ninety three USAAF P-51 Mustangs were given to the RAF. These carried four 20mm cannon and were designated as Mustang IAs.
The British placed two orders for Mustang Is. The first, for 320, was placed on 29 May 1940 and was the first production for the new aircraft. The second was for another 300 aircraft, and brought the total number of Mustang Is to 620. Of these, two were retained by the USAAF, as agreed when North American were given permission to export the aircraft. Another 93 Mustang IAs were also received under the lend lease scheme.
Twenty eight RAF and Commonwealth squadrons used the Mustang I and IA, most in the Army Cooperation role. This involved tactical reconnaissance and ground attack duties, for which the Allison powered Mustangs were ideal. The first RAF squadron to receive the Mustang was No. 2, which received the new aircraft in April 1942. The first operational sortie, an attack on a German airbase in France, took place on 10 May 1942. A more significant first came on 27 July, when Mustang Is of No. 2 Squadron became the first single engined allied fighter aircraft to enter German airspace since the French campaign of 1940. Their mission was an attack on targets in the Ruhr.
North American P-51/ Mustang I from the left