240 mm howitzer M1

A pair of three-axle, six-wheeled transport wagons were developed – one for the cannon tube, and one for the carriage. These transport wagons were also used with the 8-inch gun. These were standardized as the M2 and M3. The M2 wagon carried the carriage and the M3 wagon carried the barrel and recoil system. This separate configuration required the use of the 20-ton M2 truck-mounted crane for setup. The crane also included a clam-shell bucket that was transported on a trailer and used to dig the recoil pit for the gun. It took about two hours to emplace the gun.[7]

The transport wagons were initially intended to be towed by the Mack NO 7+1⁄2-ton heavy truck. Because the truck was a wheeled vehicle, it had problems in soft ground such as mud due to its high ground pressure. 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, so the Ordnance Department improvised in the meantime. Surplus M3 Lee-based M31 and M32B1 tank recovery vehicles and M10A1 tank destroyers were quickly modified and tested. These proved to be satisfactory and were adopted for use as the M33, M34, and M35 Prime Movers, respectively.[8] Likewise, the wheeled transport wagons loaded with the heavy gun components often became bogged down in soft ground, so the Ordnance Department developed the T17E1 carriage transport wagon with a pair of tracked bogies, and the T16E1 gun transport wagon with three medium tank bogies on each side. One 240 mm howitzer battalion was deployed to the Pacific in 1945 equipped with these carriages.[9]

The first operational use of the super-heavy 240 mm howitzer was by the 697th and 698th Field Artillery Battalions of the U.S. 5th Army at the Anzio Beachhead in Italy in January 1944.[5][10] The 240 mm howitzers and the 8-inch (203 mm) guns of Batteries A and B, 575th Field Artillery Battalion (attached to the 697th and 698th, respectively), were used as counter-battery fire against German guns in their class.[5] Their fire was accurate enough not only to knock enemy artillery out of action, but also to destroy targets as small as German heavy tanks.[5] US super-heavy artillery also played a role in countering the infamous Anzio Annie railway gun and putting it out of action.[5] In Italy, the 240 mm howitzer was highly valued for its capability to destroy key bridges at long range.[11] Army ordnance officers credited the "devastating fire power and incredible accuracy" of the 240 mm howitzer in playing a decisive role in the Italian Campaign.[1] During the Battle of Monte Cassino, the weapon was used in the final destruction of the monastery, already damaged by air attacks.[11] In the Italian theater, a small number of 240 mm howitzers also saw action with the British Eighth Army.[4] Despite their initial misgivings over the deployment of such a heavy and unwieldy weapon, 5th Army commanders dubbed the 240 mm howitzer as "the most generally satisfactory weapon" in service in 1944.[11]

By November 1944, however, all 8-inch guns and 240 mm howitzers had been withdrawn from Italy and sent to the Western Front. The 240 mm howitzer saw extensive service in Europe, reducing reinforced targets requiring super-heavy artillery.[11] Fifteen 240 mm howitzer battalions served in the European Theater; the 265th, 266th, 267th, 269th, 270th, 272nd, 277th, 278th, 538th, 539th, 551st, 552nd, 553rd, 697th, and 698th. Five served in the Pacific; the 543rd, 544th, 545th, 778th, and 779th.

During the Korean War, twelve 240 mm howitzers were brought out of "mothballs" and sent to the front lines to deal with deep bunkers and fortifications built by the Chinese that could not be effectively attacked by the smaller artillery weapons then on hand. They were utilized in two units in Korea, the 213th and 159th Field Artillery Battalions. The weapons went into action on May 1, 1953. On that day, the first round was fired by Baker battery of the 213th at a target on top of a hill called "the donut" by aerial observers. The first round was supposed to be just a ceremonial shot; however, it struck an ammo dump directly on top of "the donut" which set off a chain reaction and blew part of the top of the hill off in a spectacular fashion.[12]

In the late 1950s, several M1s were transferred to the Republic of China and stationed in the front-line Kinmen and Matsu Islands. The guns were put on rails to enable the howitzers to deploy outside for fire missions and to retreat inside their hardened bunkers/tunnels for protection (capable of withstanding direct hits from aerial bombs) when needed.[13][14]