The changes resulted from experiences with the several thousand MP38s in service since 1939, used during the invasion of Poland. The changes were incorporated into an intermediate version, the MP38/40, and then used in the initial MP40 production version. Just over 1 million would be made of all versions in the course of the war.
The MP40 was often called the 'Schmeisser' by the Allies, after weapons designer Hugo Schmeisser. Hugo Schmeisser himself did not design the MP40 but held a patent on the magazine. He designed the MP41 which was a MP40 with an old-fashioned wooden rifle stock and a selector. The MP 41 was not a service weapon in Germany.