The Panzer III's 37 mm gun was considered underpowered, yet it was used in the interests of standardisation with the infantry. Contracts for the "Zugführerwagen" (ZW, "platoon leader vehicle") were issued late in 1936. Its weight was limited to a maximum of 24 tons, because it was required to be able to cross existing bridges. Development work continued until 1938 when the Ausf D went into limited production. This 19 ton machine was powered by a 12-litre 320 hp (239 kW) engine. It had a top speed of 25 mph (40 km/h) and was fitted with 30 mm armour all round.