 Though the original T18E1 prototype was unarmed, the high cupola was replaced with a variety of machine gun mounts before the M13 cupola, with a .50 caliber machine gun, was evaluated.The prototypes were originally powered by a six-cylinder Continental AO-895-2 air-cooled gasoline engine, which exhausted through the hull side grills.This was later replaced with the AO-895-4 in the T18E1, which exhausted through a pipe mounted horizontally across the front of the vehicle.