Germany developed its first 37mm (or as German designations put it, 3.7cm) antitank gun in the last months of World War One; a very simple design built using barrels from obsolete Hotchkiss revolving cannons. In the mid 1920s, the concept was reinvigorated by Rheinmetall, which developed a much more modern 37mm gun using a much larger and higher-pressure case (37 x 249mm). In its initial pattern, the Tak as it was called (Tankabwerhkanone) has wooden spoke wheels and non suspension, intended for transport by horse. A small number were made for the German military (to avoid aggravating the conditions of the Treaty of Versailles), and the gun was sold commercially by Rheinmetall. The Soviet Union in particular was interested, and used the 3.7cm Tak as the basis for its later 45mm antitank gun.
Between 1934 and 1936, a number of changes were made to the design. The wooden wheels were replace with steel wheels and pneumatic rubber tires, and spring suspension was added to allow the gun to be towed at higher speed by motor vehicles. The designation also changed at the time from tank-abwehrkanone to panzer-abwehrkanone, as the new term “panzer” entered the German military parlance. Under Germany’s 1930s rearmament program, the Versailles limitations were ignored, and Pak production took off. By September of 1939, the German military has some 11,200 of the guns in service. They were solid dependable guns at the beginning of World War Two, but became obsolete by 1941 as tank armor began to rapidly increase. A new tungsten-core AP round was introduced in 1940, and a rfile-grenade-like hollow charge munition (the stielgranate 41) added in 1941, but there was stopgaps at best. The last few 3.7mm Paks were built in early 1942, as both it and the 50mm Pak 38 were phased out in favor of the 75mm Pak 40 antitank gun.
There is an excellent article on the development and use of the 3.7cm Pak at Tank Archives.
I’d like to thank DriveTanks.com for giving me the opportunity to bring you their 3.7mm Pak! They have a very cool collection of armored vehicles, artillery, and machine guns available for rental in Texas; check them out!