They were produced by mounting the 25 pounder gun-howitzer on an American M3A5 Grant tank hull, and were converted by the Ordnance Factory in Bendigo from 1950 to 1952.

One of the lessons of the war in Europe had been the need for artillery units in armoured brigades to have the same mobility and protection as the tanks they were supporting, and consequently a program was begun to acquire a self-propelled gun.

[1] However, as modern self-propelled artillery could not be purchased from overseas, it was decided to adapt a small number of M3 Grant tanks to the role.

[2] Drawings for the Canadian Sexton self-propelled 25 pounder—also derived from the M3 chassis—were obtained and adapted, with the turret and much of the frontal armour of the Grant replaced by an open-topped fighting compartment.

A QF 25 pounder Mk 2/1 with a 20 in (510 mm) fixed length recoil system and a muzzle brake was mounted centrally in the superstructure, allowing the gun to traverse over a 40 degree arc.

[2] In July 1949, approval was given to convert one Grant to the self-propelled configuration and the prototype subsequently underwent user and firing trials at Puckapunyal in December 1949.