After the Spanish Civil War, the German authorities started to think that a new anti-tank gun would be needed, even though the 3.7 cm Pak 36 had proven to be very successful.

[5] Rheinmetall-Borsig were forced to create a new gun with a longer - L/60 - barrel which was approved for mass production in 1939 under the designation Pak 38.

The original tank gun for the Panzer III was the 5 cm KwK 38, which despite being the same 5 cm caliber had a lower muzzle velocity due to the shorter barrel length (L/42 calibers) and smaller cartridge.

When the Germans faced Soviet tanks in 1941 during Operation Barbarossa starting June 1941, the Pak 38 was one of the few guns capable of penetrating the 45 mm (1.8 in) sloped armor of the T-34 medium tank at close range.

The gun was also equipped with Panzergranate 40 APCR shots with a hard tungsten carbide core, in an attempt to penetrate the armor of the heavier KV-1 tank.