The Thompson submachine gun struggled to find a market when it was originally produced, with the first batch of 15,000 Colt-made guns not finally all selling until the late 1930s. By that time, the clouds of war were gathering, and demand for submachine guns finally began to really grow. The US military had some Thompsons, and the British began buying as many as they could. The US wanted to increase production, and that meant simplifying the gun, both to reduce cost and to increase manufacturing efficiency. Talks to this end began in late 1941, and by February 1942 the engineers at Savage had a prototype of what would become the M1 Thompson.

This new version simplified almost every element of the gun, but most significantly it replaced the 3-piece Blish lock bolt with a solid one-piece affair that just worked as a normal blowback action. Unnecessary elements like the vertical front grip, Cutt’s compensator, quick-detach stock, and fancy contoured selector levers were discarded. The adjustable Lyman rear sight was replaced by a single metal tab with an aperture (quickly given a set of protective wings though, as the tab alone proved too fragile). The recoil guide rod was simplified, the oiling pads inside the receiver removed, and a simpler recoil buffer designed. The capability to use drum magazines was also discarded, and a new 30-round box magazine took their place.

The M1 was adopted in the spring of 1942, and July saw the first major delivery, of 48,000 guns. Simplifiecation work continued, however, and by the end of October a yet-simpler M1A1 pattern was adopted. This model replaced the hammer mechanism with a fixed firing pin. As a result, M1 production lasted only about 5 months. A total of 285,480 M1 Thompsons were made, but most of these were retrofitted to M1A1 configuration by simply swapping in the simpler new bolt. Finding intact M1 configuration guns is rather unusual today as a result.