The Panzer II was designed before the experience of the Spanish Civil War of 1936-39 showed that shell-proof armor was required for tanks to survive on a modern battlefield as prior to that, armor was designed to stop machine gun fire and shell fragments. Production began in 1935, and by July 1937, the Panzer II was cleared and ready for production and by 1939, some 1,226 Panzer IIs were in circulation.