The Thompson Submachine Gun, popularly called the Tommy Gun, was designed by General John T. Thompson, who was inspired by the trench warfare of World War I to develop a "one-man, hand-held machine gun", firing a rifle caliber round. However, the recoil from rifle rounds was too great and so it had been found that the only cartridge then in U.S. service suitable for use with the lock was the .45 ACP (Automatic Colt Pistol). The project was then titled "Annihilator I", and by 1918, most of the design issues had been resolved. However, the war ended before prototypes could be shipped to Europe.
Thompson brought the gun into production in 1921 for civilian sales, especially for police forces. It became infamous during the Prohibition era and was used by the U.S. Army during World War II.
The Thompson submachine gun or Tommy gun was the U.S. response to the Confederate automatic weapons issued to their infantry. It was a big, heavy, brutal looking weapon with a slower rate of fire than its Confederate equivalent. Nevertheless, with its .45 caliber bullets, it was an effective and robust weapon. However, U.S. soldiers continued to scavenge Confederate weapons and especially their automatic rifles throughout the Second Great War.