The FIAT submachine gun was designed by Abiel Revelli, and was developed jointly between FIAT and Officine di Villar Perosa (OVP). It was built from Revelli's earlier twin-barreled Villar Perosa submachine gun, which FIAT and OVP had also collaborated on. In 1916, Revelli demonstrated the FIAT SMG to the Italian Army, who expressed an interest in the concept and commissioned several other manufacturers to produce similar prototypes for military trials.
The FIAT was trialed in 1917 against two other competitors: the Crocetti and the Revelli-Beretta. The Beretta gun was deemed to be the best and was officially adopted by the Italian Army in early 1918. Despite having lost out, development of the FIAT gun continued at OVP after the war and the gun evolved into the Revelli Mod.1921.
The FIAT was a blowback-operated submachine gun adapted from the Villar Perosa. The entire receiver was taken from the Villar Perosa, with the only modification being the addition of a fire selector. It ejected from underneath and a case deflector was fitted to the ejection port to ensure the spent casings did not drop directly onto the user. The gun loaded from the top and used 25-round magazines; the sights were offset to ensure the magazine did not interrupt the user's aim.