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The Bison was an Improvised fighting vehicle frequently characterised as a mobile pillbox. Bisons were produced in Britain during the invasion crisis of 1940-1941. Based on a number of different lorry chassis, it featured a fighting compartment protected by a layer of concrete. Bisons were used by the Royal Air Force (RAF) to protect aerodromes and by the Home Guard. They acquired the generic name "Bison" from their main manufacturer.