In 1939 HWaA (Hitler's army Weapons command) issued a contract for the development of a "Maschinen karabiner", or machine carbine (MKb for short), chambered for the new 7.92x33mm Kurz cartridge, to the company C.G. Haenel Waffen und Fahrradfabrik. In 1940 another company joined in the development of this new type of small arm; the famous German arms manufacturing company Carl Walther, known for its fine and popular pistols. Walther had already been engaged in the development of intermediate-cartridge firearms since 1936, when it produced self-loading carbines for an experimental 7x39mm cartridge. Later, Walther developed several automatic designs in "full-size" 7.92x57mm, and one of these experimental prototypes, the 7.92mm A-115, served as a starting point for its 7.92mm Kurz rifle. Walther began to develop its own Maschinenkarabiner as a private venture, but in 1941 received official approval from HWaA for further development in competition with Haenel, the first MKb.42(W) rifles being delivered to the army in the second half of 1942.
In late 1942, the first small batches of both Haenel and Walther weapons, designated MKb.42(H) and MKb.42(W) respectively, were sent to the Eastern front, for trials against Soviet troops. Initial results were promising, with the Haenel rifles being generally preferred due to their better reliability. The Walther design, which showed better single-shot accuracy, was rejected as unsuitable on the grounds of its questionable annular gas piston system. No further development in this field was apparently taken by the Walther organization, which was already very busy delivering its P.38 pistols to the German army.
The MKb.42(W) is a gas-operated, magazine fed weapon. The gas system has an annular gas piston, located around the barrel, inside the stamped annular hand guards. A rotating bolt of somewhat complicated design locks to the barrel via two lugs. The hammer-fired trigger unit allows single shots or fully automatic fire, and the MKb.42(W) is fed using the same 30-round magazines as its rival, the MKb.42(H). The MKb.42(W) fires from a closed bolt.