Pzkpfw II was further improved and a series of new ones of the c type were produced. The c-type tanks used 5 large sized road wheels suspended only at one end by leaf spring. This suspension system newly introduced became a common feature to all the pzkpfw II produced ever since. Also, the front portion of hull was reshaped into a more roundish form. From 1937 to the beginning of 1940, Pzkpfw II of mass production types, A, B, and C, were manufactured in great numbers. To increase their productivitys, the frontal armour plate was replaced with an angular single-piece one. Pzkpfw II with specifications described above played the key role in the Blitzkrieg. At the start of this Blitzkrieg, the German tanks numbered 2580 in all, of which nearly half amounting to 955 in number were the Pzkpfw II. In 1940, mass production of the F type was started. The new Pzkpfw II was reinforced in its frontal armour (thickened to 35mm) and side (to 20mm).