The T19 Howitzer Motor Carriage variant of the M3 Half-track was built by Diamond T in 1941-2. Only 324 were ever built. It was an expedient version mounting a 105 mm howitzer in an almost standard M3: the chassis was strengthened to take the extra loads, the windshield was hinged to fold forwards, a platform was built to take the gun and the fuel tanks were moved to the rear. In use it was found to be overloaded and very cramped, so much so that only eight rounds of ammunition could be carried.
The armoured body offered little protection and there was no overhead cover. Despite these drawbacks the T19 allowed the US army to field a howitzer motor carriage in time for the North African campaign. It served as a stop-gap until the M7 Priest was ready. It was never standardized, that is it always retained its T19 designation and was never given a 'M' number. It saw active service in North Africa and Sicily and a few are believed to have soldiered on into Italy.
When the M7 Priest appeared during 1942 there was no longer a need for the T19, and therefore those that had been shipped overseas and not destroyed in combat were eventually converted back to M3's.