The British "Christy" tanks: the A.13 family

Christy's tanks revolutionized tank design. His designs were adopted by the British and Russians in large scale. The Americans just toyed with the ideas, and abandoned them (perhaps because Christy was an American). The first British Christy tank, the A13 Cruiser Mk. III was delivered in December 1938. The original Cruiser Mk.III lacked the spaced armour on the turret, but the Cruiser Mk.IV had it, although it was still using the obsolescent water-cooled machine guns. The Cruiser Mk.IVA had the more modern aircooled machine guns that were used thereafter in British tanks (at least during WWII). The A15 Crusader was a more advanced development, and had five instead of four road wheels, like the Russian T34. The A15 was better, but was still not up to world standard. It was only with the Cromwell (based on some earlier developments) that they started to be viable. It wasn't until the Comet that the type was fully developed, and by then, it was obsolete, as the breakthrough Centurion design was in prototype stage.