Changes to the specification meant that the first pilot – the 57 mm Gun Motor Carriage T49 – was built with the British (57 mm) QF 6-pounder gun instead of the 37 mm and a torsion bar suspension instead of the Christie suspension. It was tested in 1942 but the army wanted a heavier gun – the same 75 mm gun M3 as used on the M4 Sherman medium tank. The T49 project was cancelled and the second pilot was built with the 75 mm gun as the 75 mm Gun Motor Carriage T67. This met approval, but in early 1943 the army requested a more powerful gun – the 76 mm gun M1 under development for the Sherman. Six pilot models – as the 76 mm Gun Motor Carriage T70 – were built with this gun. The trials of these models led to a new turret and changes to the hull front, but the design was otherwise accepted for production, which began in July 1943.