The T18 heavy armoured car prototype was developed in 1942 by the Yellow Truck & Coach Company as a large all-wheel-drive, eight-wheeler with the front four wheels steering. Its thick armour was unusual for an armoured car, and brought the weight of the vehicle up to 26 tons - similar to contemporary medium tanks such as the M3 Lee and three times the weight of the equivalent German eight-wheeled armoured cars.
In keeping with its size, the originally specified armament of an M6 37mm gun was replaced with the newly-designed M1 57mm gun, a US-manufactured variant of the British 6 pdr gun. While the United States Army was not interested in the vehicle, the successful use of armoured cars by the British led them to place an order for 2500 T18E2 armoured cars, which they promptly christened the Boarhound. High production costs led to cancellation of the initial order after only 30 were delivered.