The Tank, Infantry, Mk IV (A22) Churchill was a British heavy infantry tank used in the Second World War, best known for its heavy armour, large longitudinal chassis with all-around tracks with multiple bogies, its ability to climb steep slopes, and its use as the basis of many specialist vehicles. It was one of the heaviest Allied tanks of the war. The origins of the Churchill's design lay in the expectation that war in Europe might well be fought in conditions similar to those of the First World War, and thus emphasised the ability to cross difficult ground. The Churchill was hurried into production in order to build up British defences against a possible German invasion. The first vehicles had flaws that had to be overcome before the Churchill was accepted for wide use. After several Marks (versions) had been built, a better-armoured specification, the Mark VII, entered service with the British Army. The improved versions performed well in the later stages of the war. The Churchill was used by British and other Commonwealth forces during the North African, Italian and North-West Europe campaigns. In addition, 344 Churchills were sent as military aid to the Soviet Union during the Second World War and more than 250 saw active service on the Eastern Front.