In 1919, the Westervelt Board described the ideal heavy gun for future development having a bore of 194 mm to 8 inches, a projectile of about 200 lbs in weight, and a range of 35,000 yards.

Serious development restarted in June 1940, resulting in an 8-inch (203 mm) gun that would have a range of 33,500 yards (30,600 m), be transported in two loads weighing no more than 44,000 lb (20,000 kg) at a road speed of up to 25 mph (40 km/h), and also be suitable for rail movement.

The main problems were determined to be excessive bore wear, leading to poor accuracy, but it was felt that nothing better could be produced in a timely manner.

Like the 240 mm howitzer, it was mounted on a stretched Heavy Tank T26E3 chassis that had an extra bogie wheel per side as the prototype 8-inch gun motor carriage T93, but the war ended before they could be used, and were later scrapped.

Eventually, a pair of three-axle, six-wheeled transport wagons were developed – one for the barrel and recoil mechanism, and one for the carriage.

The M6 high-speed tractor, a tracked vehicle which was explicitly designed for towing the 8-inch gun and 240 mm howitzer, was standardized in June 1943 but would not enter production until February 1944.

[2] The M35 Prime Mover (not to be confused with the M35 truck) was appreciated for providing superior mobility and speeds, and would be the primary transport vehicle used in Europe until the surrender of Nazi Germany to the Allies.

[6] Battery A of the 575th also went to the Cassino front attached to the 697th Field Artillery Battalion, and was used in the counter-battery role against long-range German 170 mm guns.

By September 1944, the 8-inch guns of the 575th had been withdrawn from Italy, and soon saw action in France where they were particularly effective against fortified targets and in counter-battery fire against German long-range artillery.

During the Battle of Saint-Malo, two batteries of the 243rd Field Artillery Battalion of 8-inch guns participated in the siege in support of the 83rd Infantry Division.

[8] In the Siegfried Line campaign, in support of the American 3rd Infantry Division, the 8-inch guns adjusted by aerial observers, were successful in destroying two bridges over the Roer River.