Concept
The 75 mm gun, models M2 to M6, was the standard American medium caliber gun fitted to mobile platforms during World War II. They were primarily mounted on tanks, such as the M3 Lee and M4 Sherman, but one variant was also used as an air-to-ground gun on the B-25 Mitchell medium bomber aircraft. There were five main variants used during the war: M2, M3, M4, M5 and M6. They were considered the standard American tank guns. The M2 and M3 were used on the M3 medium tank, the M3 was used on the M4 Sherman tank, and the M6 was used on the M24 Chaffee light tank. The M3 was also used on Medium Tank M7. The M4 variant was fitted on some North American B-25 Mitchell medium bomber aircraft. The 75 mm tank gun has its origins in the January 1937 specification for a light anti-aircraft gun T6 which should have supplemented heavy 3-inch guns and shoot the same range of 75x350R ammunition that 75 mm field gun M1897 shot. After the design, featuring a 31-caliber barrel and sliding block breech, failed the trials, it was reused to develop T7 tank gun in mid-1940. British tanks in the early years of World War II relied on high-velocity ordnance derived from anti-tank guns, such as the 40 mm calibre Ordnance QF 2 pounder and, later, 57 mm calibre Ordnance QF 6 pounder, for their primary armament. According to the Royal Armoured Corps doctrine, they were not supposed to fire HE shells, which turned out to be a great disadvantage. Post-war, the UK's Tank Museum credited the US 75mm gun as "America's most important contribution to tank warfare" because of its ability to combine good (for the time) AP and HE performance. They believed that the American observers working with the British before Dunkirk had appreciated the effectiveness of such a dual purpose weapon, and so it was made a prime requirement for US tank production. After experiencing the effectiveness of the American 75 mm tank guns in the infantry support role, the British opted to adopt the American caliber and ammunition by the expedient of boring-out the 6 pounder tank gun to make the Ordnance QF 75 mm.
