The Churchill Mk. I is a British infantry tank that was designed to support infantry attacks, serving throughout the Second World War. It was one of the heaviest Allied tanks of the war, with a weight of 28 tons and a thick armour which could hardly be penetrated by Comintern cannon fire at range. It had a crew of five and was armed with a two-pounder (40mm) gun in the turret and a 3-inch (76.2mm) howitzer mounted on the hull. The howitzer was intended to fire high-explosive shells at enemy fortifications, while the gun was used against other tanks. It was first used in the Lowlands War, providing much-needed armoured support to British Expeditionary Force infantry against French forces. Despite its heavy armour, the Churchill Mk. I suffered many losses and proved to be unreliable and slow. It was soon replaced by improved versions, such as the Churchill Mk. III and Mk. IV, which had a more powerful gun and better engine. The Churchill platform was also the basis of many specialist vehicles, such as flamethrowers, bridge-layers, and mine-clearers.