Heavily inspired by the successful Swedish Bofors L/60 40mm AA design of the late 30s, Zavod #8 "Kalinin" developed a new gun for the divisional air defense units of the Red Army.
The production start of the new 61-K M1939 still in 1939 was the birth of one of the most successful Russian guns of the upcoming Great Patriotic War.
The 61-K excelled by high muzzle velocity, rate of fire and maximum effective ceiling. It was simple and rugged as was the carriage. Some guns carried shields for the crew which normally consisted of a commander and seven men.
In order to improve mobility the M1939 was occasionally mounted on ZiS-5, ZiS-6 trucks as well as on ZiS-42 haltftracks. Later in war, an AA SPG based on the SU-76 was produced as the ZSU-37.
It comes as no surprise that the German army took captured examples into their armoury as the "3.7 cm Flak M.39a (r)".
No less than 9,100 guns were produced and remained in service well into the 80s of the last century.