The conclusion of the board vis-a-vis corps (heavy field) artillery was that an ideal heavy howitzer should have range of at least 16,000 yards (15 km) and allow the elevation of 65°[2] (as opposed to the existing World War I-era M-1918 155 mm howitzers, a license-built French Canon de 155 C modèle 1917 Schneider, 11.5 km and +42° 20' respectively).

[3] By 1934, the US Army was concerned about the arising high-speed towing requirements not satisfied by the plain bearings and solid rubber ties.

[4] In 1939 the development began anew,[5] by spring 1941 the first specimen was ready to be test-fired and immediately after passing them[6] it was standardized on 15 May 1941 as Howitzer M1 on the Carriage M1.

The M1A1E1 carriage was intended for use in jungle and muddy terrain and replaced the wheels of the M1A1 with a free-wheeling tracked suspension, but the project was terminated after V-J day without having reached production.

The T-16 was a light-weight carriage using high-grade steel that was estimated to save some 1,200 lb (540 kg); work began in July 1945 and continued after the war, although nothing seems to have come from it.

The XM123 was produced by American Machine and Foundry and outfitted with two 20 horsepower air-cooled engines produced by Consolidated Diesel Corporation, driver's seat, steering wheel, and guide wheel on the left trail, allowing it to be more rapidly emplaced when detached from the prime mover, while the XM123A1 provided a single 20 horsepower motor with electric steering.

The extra weight on the left trail displaced the howitzer after each round was fired, requiring it to be realigned, and the project was abandoned.