The first iteration of the iconic German Sturmgewehr was developed by Haenel starting in 1938. It was a select-fire rifle chambered for the short 8x33mm cartridge, developed by the Polte company. It used a long-stroke gas piston and a tilting bolt patterned after the Czech ZB-26 light machine gun. What makes the MKb42(H) stand out from the later Sturmgewehr models is that it was an open-bolt design. The original design spec was concerned about preventing cook-offs, and so it required firing from an open bolt. This means a very simple fire control system, but it also made the rifle difficult to shoot accurately in semiautomatic.

The first MKb42(H) prototype was finished in 1941, with 50 sample guns produced by late March 1942. A major trials was held inApril 1942, in which Hitler rejected the design (mostly, he disliked the smaller cartridge). Development was continued anyway, with a move to a closed-bolt system that would become the MP43/1 which was ready for its first testing in November 1942. The open-bolt 42(H) was put into production anyway, as a stopgap measure to provide some much-needed individual firepower to troops on the Eastern front. Serial production began in January 1943, and continued until September 1943. In total, 11,813 of the rifles were manufactured. They saw use in Russia until replaced by newer MP43 models, and represent the first combat use of the assault rifle concept.

Video on the Walther MKb42(W):
https://forgottenweapons.vhx.tv/videos/mkb-42-w-the-sturmgewehr-that-never-was