This machine pistol was designed by a French engineer, Louis Bonnet de Camille, who had previously designed the MGD PM-9. In 1955, de Camille's financial sponsor, Fenner Achenbach, licensed both of the designs to Erma in an attempt to get them a manufacturer. Erma only briefly took up production of the PM-9 before abandoning it, and later in 1956 they produced prototypes of de Camille's machine pistol, designating it the MP56. However, the newly-reestablished Erma lacked the industrial means to produce the MP56 in any large numbers, so the prototypes were made by hand. Only ten were made before Achenbach handed over production rights to Mauser, who produced their own similarly unsuccessful version known as the MP57.
The MP56 was an Uzi-like blowback submachine gun that utilized a "wrap-around" bolt. It cocked from the sight side of the receiver. The initial prototypes of the MP56 had collapsible stocks and grip safety, features that were dropped on subsequent prototypes in favor of a folding grip and manual safety. There was no fire selector, and only fully-automatic fire was possible. It fed through MP40 magazines.