The J Type was a 3-ton 4x2 truck, it was powered by a Thornycroft M4 four-cylinder inline 6,256 cc side-valve petrol engine that developed 45 brake horsepower (34 kW) at 1,800 rpm, it drove the rear wheels through a four-speed gearbox.

[1][2] The J Type was designed by Thornycroft to meet a 1911 War Office specification for a 3-ton "subsidy scheme" cargo truck, the scheme provided a grant of £110 to civilian companies towards the purchase price of trucks suitable for military service, but included a provision that the military could requisition the vehicles if they required them.

[2] In order to provide mobile anti-aircraft defence, the British Army removed the QF 13-pounder gun from its field carriage and placed it on a high-angle pedestal creating the QF 13-pounder 6 cwt AA gun, it was usually mounted it on the rear of a lorry, 183 such self-propelled anti-aircraft (AA) guns were produced by Thornycroft on J Type chassis.

[3] The 6 cwt guns were later replaced by the improved QF 13-pounder 9 cwt on the same chassis and despite being introduced as a stopgap solution it saw service on the Western Front until the end of the war, it remained in Canadian Army service until 1930.

[3] The J Type 13-pounder self-propelled AA gun combination was very popular with its crews, being renowned for their reliability, although its very poor cross country performance was considered a failing.