Specification Crew : 6 Engine power : 400 bhp Combat weight :26,650 kg (58,763 lb) Max speed : road 32 km/h (19.87 mph) Length : 6.70 m (21.98 ft) Range : 300 km (186.30 miles) Width : 3.86 M (12.66 ft) Main gun : 155 mm(6.11 in) Height : 2.69 m (8.83 ft) Armour : 9.4 mm (0.37 in)to 25.4 mm(1.00 in)

In mid-1941 Us Army Artillery staff requested a 155 mm(6.11 in)gun which could be mounted on the M3 medium tank chassis.A design using a long-barrelled 155 mm M1918 gun was obtained and despite some opposition the artillery staff persisted and an order was placed for 100 examples of the Gun Motor Carriage M12.The conversion of the M3 was straightforward:the engine was moved forward leaving the rear of the hull free for to accommodate the gun mounting and leaving space into which the breech could be lowered.The gun mounting was open but since the 155 mm gun was a long range weapon the crew would not(in theory)be exposed to enemy fire. The US Army continued to ignore the need for the M12 so the 100 built were initially used only for training within the United States.However by late 1943 with the invasion of Europe looming it was decided that there was a need for the M12 after all.A total of 75 M12s were rebuilt prior to being sent to North-West Europe.The Cargo Carrier M30 was identical to the M12 apart from the omission of the gun it was used as an ammunition carrier for M12 batteries on a one-to-one basis since the M12 could carry only 10 rounds ready to fire.Most of the gun crew also travelled on the M30.