The MP40 is descended from its predecessor, the MP38. The MP36, a prototype made of machined steel, was developed independently by Erma's Berthold
Geipel with funding from the German army. It took design elements from Heinrich Vollmer's VPM 1930 and EMP. Vollmer then worked on Berthold Geipel's
MP36 and in 1938 submitted a prototype to answer a request from the German Armament services for a new submachine gun, which was adopted as MP38.
The MP38 was a simplification of the MP36, as the MP40 was a further simplification of the MP38, with certain cost-saving alterations, notably in the use of
more pressed rather than machined parts. Other 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 MP41 was not introduced as a service weapon in Germany.

Both MP38 and MP40 submachine guns are open-bolt, blowback-operated automatic arms. Fully automatic fire was the only setting, but the relatively low rate
of fire allowed for single shots with controlled trigger pulls. The bolt features a telescoped return spring guide which serves as a pneumatic recoil buffer. The
cocking handle was permanently attached to the bolt on early MP38s, but on late production MP38s and MP40s, the bolt handle was made as a separate
part. It also served as a safety by pushing the head of handle into a separate notch above the main opening, which locked the bolt either in the cocked or
forward position. The absence of this feature on early MP38s resulted in field expedients such as leather harnesses with a small loop, used to hold the bolt in
forward position.

The receiver was originally machined steel but this was a time-consuming and expensive process. This prompted the development of a simpler version that
used stamped steel and electro-spot welding as much as possible. The MP38 also features longitudinal grooving on the receiver and bolt, as well as a circular
opening on the magazine housing. These features were suppressed on the M38/40 and MP40.

One idiosyncratic and visible feature on most MP38 and MP40 submachine guns was an aluminum or plastic rail under the barrel which was used as a
support when firing over the side of open top armored personnel carriers such as the Sdkfz 251 half-track. A handguard was located between the magazine
housing and pistol grip and was made of synthetic material derived from bakelite. The barrel lacked any form of insulation, which often resulted in burns for
the supporting hand if it strayed. It also had a folding stock, the first for a submachine gun, resulting in a shorter weapon when folded, but it was insufficiently
durable for hard use and hand-to-hand combat.

Although the MP40 was generally reliable, a major weak point was its 32-round magazine. Unlike the Thompson's double-column, dual-feed magazine, the
MP38 and MP40 used a single-feed design. The main cause of malfunction is that the magazine was also frequently misused as a handhold, which could also
cause a failure to feed when hand pressure on the magazine body caused the feed lips to move out of position, since the magazine well did not keep the
magazine firmly locked like on a Sten. German soldiers were trained to grasp either the intended handhold on the underside of the weapon or the magazine
housing with the supporting hand to avoid feed malfunctions.

Unlike the impression given by popular culture, MP40s were generally issued only to paratroopers and platoon and squad leaders; the majority of soldiers
carried Karabiner 98k rifles. However, experience with Soviet tactics where entire units armed with submachine guns outgunned their German counterparts in
short range urban combat caused a shift in tactics, and by the end of the war it was being issued to entire assault platoons on a limited basis.

There were never enough MP40s because raw material and labor costs made it expensive to produce alongside the Kar98 rifles. Due to this, starting in 1943,
the German army moved to replace both the Kar-98 rifle and MP-40 with the new MP-43/44 assault rifle, also known in its production model as the StG44.