British local industry built several major improved fighting vehicles for local defense in the event of an Axis invasion of the British mainland. This threat seemed very real after the Fall of France in May of 1940 and it was realized that the homeland lacked true defensive measures against an all-out ground or air assault. This led to companies taking very standard lorries (trucks) and adding all sorts of protection and arms to them - resulting in fighting vehicles that held little military value but assuaged civilian fears of a direct invasion. At their core, the vehicles were nothing more than armored, mobile pillboxes.
Thornycroft was one of the British concerns to work on several armored lorries during the period and one of its creations was the "Bison Concrete Armoured Lorry" to serve with British Home Guard units. It arrived in 1940 with design attribution given to Charles Bernard (C.B.) Mathews who headed Concrete, Limited.
The "Bison" name went on to identify several similar-minded designs as there proved no standard for the vehicle - any truck chassis could form its basis so long as it held the power and strength to haul a heavyweight superstructure. This often meant that engines were of whatever make and model was available to the original chassis and wheel arrangements could vary between two or three axles. Standard designs sat concrete slabs over key sections of the truck - such as the engine block and crew spaces. For the crew within, vision slots were provided for engaging enemy elements through rifle fire and pistols. There were hatches for entry-exit of passengers and operating crew. Concrete was used due to the unavailability of more valuable metals in wartime.
In the end, the invasion of the British homeland never took place and Bison lorries were relied upon for more basic second-line duties such as serving in defense of RAF airfields for the remainder of their days. Since the heavy vehicles lacked strong off-road performance, it played well to the Bison's strength that it operated on level ground.
Three major Bison types were identified - Type 1 was the lightest form and fitted a canvas-topped armored cab while Type 2 was similar but dimensionally larger and carried a separate crew compartment aft. The Type 3 was the largest offering - and consequently the heaviest - and integrated the crew compartment with the driver's compartment for improved communications.
In any form their tactical value and usefulness was always in questioned - though their psychological effect on British civilians, thanks in large part to local propaganda, was well-known.