In the early 1920s, Vollmer started to develop his own sub-machineguns. His early models, named VPG, VPGa, VPF and VMP1925 were fairly similar to the MP18. The VMP1925 had a wooden handgrip and was fed by a small 25-round drum magazine, connected directly to the gun. The VMP1925 was secretly tested by the Reichswehr, along with competing designs from Schmeisser and Rheinmetall. (The Reichswehr was prohibited by the Versailles Treaty from having sub-machine guns in service, although the German police was allowed to carry a small number.) Secret funding was given to Vollmer to continue development, and this resulted in the VMP1926, which mostly differed from its predecessor by the removal of the cooling jacket. A subsequent development was the VMP1928, which introduced a 32-round box magazine sticking from the left side. The final development of this series was the VMP1930. (It can also be seen at the WTS.) This model introduced a substantive innovation—a telescoping main spring assembly, which made the gun more reliable and easier to assemble and disassemble in the field. Vollemer applied for a patent for his innovation in 1930 and it was granted in 1933 as DRP# 580620. His company, Vollmer Werke, produced however only about 400 of these, and most were sold to Bulgaria. In late 1930, the Reichswehr stopped supporting Vollmer financially; consequently he sold the rights to all his designs to the company known as Erma Werke (which is an abbreviation for Erfurter Maschinenfabrik, Berthold Geipel GmbH).
The submachine guns that Erma started to sell in 1932 under the names EMP (Erma Maschinenpistole) or MPE (Maschinenpistole Erma) was basically just the VMP1930 with the cooling jacket restored. Although there were several variants with varying barrel lengths and sights made to customers' specifications, roughly three main variants were produced: one with a 30 cm barrel, tangent rear sight and bayonet lug was apparently sold to Bulgaria or Yugoslavia. The second model, sometimes called the MP34, or the "standard model", had a 25 cm barrel and no provision for a bayonet; the rear sight on these varies—some had a tangent sight, others a simplified flip-up "L" sight. A third variant was basically similar in the metallic parts, but replaced the foregrip with a MP18-style stock with finger-grooves. Overall, at least 10,000 of these Vollmer-based designs were made by Erma. They were adopted by the SS in 1936, but also sold to South-American countries and to Spain, where it was subsequently manufactured locally under the designation M41/44.
In the Spring of 1939, a large number of defeated Spanish Republicans fled to France, where they were disarmed. Some 3,250 EMPs formerly in the possession of these fighters ended up in a French warehouse at Clermont-Ferrand. The EMPs were usually referred to as the "Erma–Vollmer" in French documents. The French tested the weapons and decided to adopt them for their own service. A provisional manual was printed in French as Provisoire sur le pistolet-mitrailleur Erma – Vollmer de 9mm, issued on December 26, 1939 and updated in January 6, 1940. However, the French had obtained only some 1,540 suitable magazines for these guns, so only 700-800 EMPs were actually distributed to the French forces, mostly to the Mobile Gendarmerie. After the Nazi conquered France, some EMPs armed the Legion of French Volunteers Against Bolshevism, which eventually became part of the SS Charlemagne division. This division was practically destroyed in February 1945 in Eastern Prussia, now part of Poland. Numerous EMPs have been found in the last-stand battlefields of the SS Charlemagne division; most of these guns lack any German military stamps or marks. The EMPs which arrived in German hands via the French route were given the (Fremdgerät) designation 740(f).
In Francoist Spain, the EMP was produced chambered in the 9mm Largo cartridge. It was informally known as "subfusil [modelo] Coruña".