The Prime Mover M33 was an artillery tractor based on the M31 Tank Recovery Vehicle, itself based on the Medium Tank M3.
The first effort to produce a prime moved based on the M3 produced the T16, which was built around M3A5, but this lacked storage space and the project was abandoned.
A more dramatic conversion saw the M3 turned into the M31 Tank Recovery Vehicle. This involved removing the 75mm and 37mm guns and related equipment, installed a crane in place of the 37mm gun and greatly increasing the amount of storage available. A total of 805 M31s were produced by converting surplus M3s, M3A3s and M3A5s.
The M31 became the basis of the M33 Prime Mover. These had the crane, 37mm turret, internal winch and machine guns removed and a compressor installed to operate the artillery brakes. The turret ring was left open. One machine gun could be carried on a ring mount installed above the 75mm gun rotor (and somewhat resembling the machine gun pulpit of the M7 'Priest'. A total of 109 M31s were converted into prime movers as the M33, along with twenty four M4s, which became the M34.