Hitler's drive to modernize his armed forces gained a new momentum with the arrival on stage of Col. Heinz Guderian - the future spiritus movens of German armored warfare doctrine. Behind the scenes German design teams were busy working on prototypes of vehicles that would soon become the tools of the future war - light Pz.Kpfw. I and II, heavy (in keeping with contemporary classification) Pz.Kpfw. IV and medium Pz.Kpfw. III armed with a 37 mm gun. In the early stages of fighting in France it became clear that the vehicle didn't carry enough punch and in later marks of the tank the 37 mm main gun was superseded by a 50 mm weapon. The ultimate version of the Pz.Kpfw. III was armed with a short barrel 75 mm gun, the largest that the tank's turret could accommodate.