Pre-war plans called for two main tanks: the main tank was to be the Panzer III medium tank, supported by smaller numbers of the howitzer-armed Panzer IV. However, by the beginning of the invasion of Poland, only a few hundred of these vehicles were available. As a result, the invasions of Poland and France were carried out primarily with the less capable Panzer I and Panzer II light tanks (armed with machine guns and a 20 mm gun respectively), with some gun-armed light tanks of Czechoslovak design (Panzer 35(t) and Panzer 38(t), both armed with a 37 mm gun). Even in 1941, Panzer III production amounted only to about a thousand tanks, forcing the Germans to use Czech tanks as substitutes for the Panzer III. As the war proceeded, production of heavier tanks increased.