while Krupp's turret was more rectangular and had the 3.7 cm gun mounted beside the 7.5 cm gun. Both turrets were also armed with a co-axial MG 34 machine gun, along with the two sub-turrets at the front and rear of the tank. The Neubaufahrzeug was intended to fulfill the role of a medium tank in Germany's developing armored force, but it proved to have too many problems with its front drive and aircraft engine for this role. But even with all its faults, the Neubaufahrzeug provided insight into tank designing that was valuable to the next German medium tank project, the Begleitwagen ("accompanying vehicle") which would come to fruition as the Panzer IV tank for infantry support. In 1934 Rheinmetall built two mild steel prototypes, one with their own turret design and one with Krupp's. Three more prototypes were built with proper armor and the Krupp turret design in 1936. The Großtraktor was later put into service for a brief period with the 1 Panzer Division; the Leichttraktor remained in testing until 1935. In the late 1920s and early 1930s German tank theory was pioneered by two figures: General Oswald Lutz and his chief of staff, Lieutenant Colonel Heinz Guderian. Guderian became the more influential of the two and his ideas were widely publicized. Like his contemporary Sir Percy Hobart, Guderian initially envisioned an armored corps (panzerkorps) composed of several types of tanks. This included a slow infantry tank, armed with a small-caliber cannon and several machine guns. The infantry tank, according to Guderian, was to be heavily armored to defend against enemy anti-tank guns and artillery. He also envisioned a fast breakthrough tank, similar to the British cruiser tank, which was to be armored against enemy anti-tank weapons and have a large 75-millimeter (2.95 in) main gun. Lastly, Germany would need a heavy tank, armed with a massive 150-millimeter (5.9 in) cannon to defeat enemy fortifications, and even stronger armor. Such a tank would require a weight of 70 to 100 tonnes and was completely impractical given the manufacturing capabilities of the day. In the early 1930s, the German Army called upon a few German firms to put together some funded prototype light and medium tanks. At this time, the Army did not have a formal plan of action in terms of what it realistically needed. Light tanks could be made available in large quantities for a relatively