The Soviet Navy purchased a number of Bofors 25 mm Model 1933 guns in 1935, trials of the weapon were successful and it was decided to develop a 45 mm version of the weapon designated the 49-K. The development under the guidance of leading Soviet designers M. N. Loginov, I. A. Lyamin and L. V. Lyuliev was successful, but the army thought that the 45 mm calibre was a little too large for an automatic field weapon. In January 1938 the Artillery Factory Number 8 in Sverdlovsk was ordered to develop a 37 mm weapon based on the same design. The task was fulfilled by the chief designer of the factory, Mikhail Loginov, and his assistant Lev Loktev. Firing trials of the new 61-K were conducted in October 1938. Competitive firing trials were conducted in 1940 between the 61-K and the Bofors 40 mm/56. There were no substantial differences found between them.