The T18 design resulted from the characteristics and capabilities of the heavy armored car as envisioned by the US Ordnance and British Armored Force Boards circa July 1941.  Two (2) pilot vehicles each of the T18 and contemporarily designed* T18E1 6x6 Armored Car were originally ordered. However very soon afterward, conditions changed and production of the 6x6 T18E1 was cancelled and only one (1) T18 prototype was actually built.  The second proposed T18 prototype was modified in a number of ways, most notably to mount a 57mm Cannon instead and re-designated as the T18E2 Boarhound Armored Car.

* The T18E1 6x6 Armored Cars were cancelled due to the development of the T19 Armored Car.