The M1935 was originally designed in 1932 by Karl Heinemann, a Mauser engineer. Heinemann sold the production rights for the gun to FN Herstal in Belgium, who produced a limited batch in 1935. It failed to sell well enough to warrant further production.
The M1935 was a recoil-operated, toggle-locked submachine gun. It used a series of two pivoting arms attached to the bolt, which would fold into a V-shape when the bolt retracted. In this state, the empty casing would be ejected and a new cartridge chambered. Then the bolt would reach a short return spring located in the rear of the weapon, and the return of the bolt would be accelerated by the force of the toggle arms pushing back on it. This mechanically complex system proved to be user-unfriendly and very expensive to produce.
The weapon fed from the left side and was offered in both 9×19mm and 7.63×25mm. It cocked from the right side and was fitted with a heavy barrel with a four-slot compensator. A double trigger was used to give fully-automatic fire and single shots.