The Lewis Gun or Lewis Automatic Machine Rifle was invented by U.S. Army Colonel Isaac Newton Lewis in 1911, based on initial work by Samuel Maclean. Despite its origins, the Lewis Gun was not instantly adopted by the Americans. This may have been due to a clash of personalities with the U.S. Army's Chief of Ordnance. It used the British .303 round and had a cyclic rate of approximately 550 rounds per minute. The gun weighed 28 lb (12.7 kg), only about half as much as a typical medium machine gun of the era, such as the Vickers machine gun, and was primarily chosen because it could be carried and used by a single soldier. The Lewis Gun was therefore more mobile than heavy machine guns and could much more easily follow the troops during advances and retreats, but it was still a bit heavy for its intended role. It was also relatively cheap at about one sixth the cost of a Vickers, and was issued in large numbers to soldiers serving on the Western Front.

    In total, only 62 parts made up the gun (six Lewis Guns could be produced in the same time as a Vickers gun due to the excessive complexity of the Vickers lockwork). The lighter weight of the Lewis Gun made it popular as an aircraft mounted weapon, especially since the cooling effect of the high speed air over the gun meant that the radiator and cooling fins could be removed, making the weapon even lighter.