Based on the M3 Half-track, the T30's specifications were similar to its parent vehicle. It was 20 ft 7 in (6.28 m) long, 6 ft 5 in (1.96 m) wide, 8 ft 3 in (2.51 m) and high, and weighed 10.3 short tons (9.3 t). The suspension consisted of vertical volute springs for the tracks and leaf springs for the wheels, while the vehicle had a fuel capacity of 60 US gallons (230 l). It had a range of 150 mi (240 km) and had a speed of 40 mph (64 km/h), and was powered by a White 160AX, 147 hp (110 kW), 386 in3 (6,330 cc), six-cylinder, gasoline engine, with a compression ratio of 6.3:1. It had a power-to-weight ratio of 15.8 hp/ton.
The T30's main armament was a short barreled 3.0 in (75 mm) pack howitzer. The 75 mm Pack Howitzer M1 as mounted could depress nine degrees, elevate 50 degrees, and traverse 22.5 degrees to each side. The vehicle had stowage for sixty rounds of 75 mm ammunition, and although it was not designed for anti-tank use, it had a high explosive anti-tank (HEAT) shell that could penetrate 3 in (76 mm) of armor at normal ranges. The gun shield had 0.375 in (9.5 mm) thick armor, designed to stop a .30 cal (7.62 mm) bullet from 250 yards (230 m) away.
The T30 HMC was originally conceived in 1941 as an interim design to fulfil the Armored Force's requirement for an assault gun to equip tank and armored reconnaissance units. The Ordnance Department design was based on the M3 Half-track in order that it could be brought into service quickly. The pilot vehicle was authorized in October 1941 armed with an M1A1 75 mm Pack Howitzer and its mounting designed to fit on a simple box structure in the back of an M3 Half-track.
Authorization for production of two pilot vehicles was made in January 1942; first deliveries of the vehicle were made the following month from the White Motor Company. As it was seen as a temporary solution it was never given type classification. In September 1942, the T30 was partially replaced by the Howitzer Motor Carriage M8 (the same gun on an M3 Stuart). After that, it was declared as "limited standard". Around 500 were produced, all by the White Motor Company.