The Bison was an extemporised armoured 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.