The Panzer III was intended to fight other tanks; in the initial design stage a 50-millimetre (1.97 in) gun was specified. However, the infantry at the time were being equipped with the 37-millimetre (1.46 in) PaK 36, and it was thought that, in the interest of standardization, the tanks should carry the same armament. As a compromise, the turret ring was made large enough to accommodate a 50-millimetre (1.97 in) gun should a future upgrade be required. This single decision later assured the Panzer III a prolonged life in the German Army.
The early Panzer III Ausf. A to early Ausf. G were equipped with a 3.7 cm KwK 36 L/45, which proved adequate during the campaigns of 1939 and 1940. In response to increasingly better armed and armored opponents, the later Panzer III Ausf. F to Ausf. J were upgraded with the 5 cm KwK 38 L/42. And the later Panzer III Ausf. J¹ to M went with the longer 5 cm KwK 39 L/60 gun.