The Villar Perosa M15 was an Italian portable automatic firearm developed during the World War I by the Officine di Villar Perosa, originally to be used by the second crew member/observer of military aeroplanes. While it was designed to use 9mm pistol ammunition, it is not a 'true' submachine gun because of its intention to be used as a mounted weapon.
The Villar Perosa was designed as a portable double barrel machine gun firing a 9mm round. In practice the machine gun consisted of two independent coupled weapons, each with its own barrel firing mechanism and separate 25 round magazine. As it was originally designed to be operated from airplanes it had a high rate of fire of over 1,500 rounds per minute. However, in practice the 9mm munition was not sufficiently powerful to shoot down aircraft which had become ever more resistant over the course of the war, and in addition the range was inadequate.
Instead the Villar Perosa began to be issued to ground troops. Between May and November 1916 a section was assigned to each infantry battalion of the Italian army and from May 1917 the number of sections was increased to 3 per battalion. The weapon was first used at the 12th Battle of Isonzo.