The M9 3in Gun Motor Carriage was a fully armoured tank destroyer that was based on the M3 medium tank chassis, but that was cancelled soon after being ordered into production.
The first attempt to produce a fully armoured tank destroyer was the T24. This carried an M3 3in anti-aircraft gun on a modified M3 medium tank chassis. The turret was removed, as was the main gun in the hull. A new superstructure was developed, with a sloped front plate. The 3in gun was carried in an open topped fighting compartment, with the gun almost entirely above the superstructure. The T24 was abandoned in March 1942 as the mount was felt to be too high.
Work then moved onto the T40. This used the M1918 3in anti-aircraft gun, again on the M3 chassis and with a similar superstructure to the T24. This time the gun was mounted lower down in the superstructure, with the barrel moving in a slot in the front plate. In December 1941 a proposal was made to build fifty T40s to use up a stock of fifty M1918 guns.
In April 1942 the order was confirmed and the T40 was standardised as the M9 3in Gun Motor Carriage. Fifty vehicles were to be built as the 'limited standard' M9 3in Gun Motor Carriage. This soon ran into problems. First the Tank Destroyer Board objected to the M9 on the grounds that it lacked mobility and speed. It was then discovered that there were only twenty eight guns available. Work on the M10 3in Gun Motor Carriage was already well advanced, and so in August 1942, before production had begun, the M9 was cancelled.