The 39.M was originally designed in the late 1930s, developed from the earlier SIG MKMO, on which Király had worked as a draftsman. In 1938 Király sent the blueprints to Birmingham Small Arms in Britain, who produced several prototypes, known as the BSA-Király. These prototypes were tested by the British Army but not adopted on account of their considerable technical complexity. In 1939, Király managed to get the weapon produced domestically by Danuvia, and it was adopted by the Hungarian Army.
Due to the technical complexity of the gun, only 8,000 were produced during World War II. A very small quantity of these were hinged-stock models known as the 39.A. In 1943, a new model known as the 43.M was produced, which had a folding skeleton stock, shortened barrel and new magazine. A simplified variant, the 44.M, was in development during the late stages of the war. Production of all versions ceased when Hungary was invaded by the Soviet Union.
The Danuvia uses lever-delayed blowback, using a patented two-part delayed blowback bolt. The fire selector is a circular cap on the rear of the receiver. The ejection port and the cocking handle are on the right of the receiver.