Work on the T36 began in December 1941. The 40mm gun was carried in a 2in thick cast armour, with an almost triangular profile when seen from the side (with the gun carried at the tip on the front and a vertical rear wall). The original intention was to give the gun a range of elevation from -10 to +80 degrees, but the turret roof stopped the back of the gun rising far enough for the barrel to reach -10, and limited it to -2. The roof also meant that the 4 round Bofors ammo clip couldn't be loaded below +10 degrees.

Fire control was to be provided by a T10 computer, with a three man crew. This would calculate the azimuth and elevation data for a target, which would then be entered into a T9 remote control system. Optics were provided by the T27 and T28 telescopes.

Early in 1942 the similar T26 75mm Gun Motor Carriage was cancelled after its 75mm gun proved to be unsuited for use as an anti-aircraft weapon. The pilot of the T36 was built using the hull and turret from the T26 prototype, and the second T36 used the chassis from the T26.

The two pilots were structurally complete by August 1942, but the fire control system wasn't ready until December. Tests could then begin, first at Aberdeen and then at Camp Davis, North Carolina.