While lighter tanks formed almost the entirety of the German forces, heavier tanks were at least at the prototype stage. In 1934, a number of heavy prototypes were constructed, based around either 75 or 105 mm main guns. Designated "Neubaufahrzeug" (NbFz) and very similar to contemporary Soviet and British designs, six were built by Rheinmetall and Krupp. Although they were mainly useful for propaganda purposes because only a handful of the tanks were made, three were actually used in combat in Norway. The knowledge of the NbFz and the experiences of the lighter tanks in Spain aided in designing the next generation of tanks, the Panzer III and Panzer IV. The PzIII was the lighter of the two and was intended for use in anti-tank operations, whereas the heavier PzIV was armed with a short-barrel gun and intended for supporting infantry.