armored LVTs, two companies of DUKWs and two battalions of cargo LVTs be assigned to each division in future amphibious operations. The number of LVTs by battalion was increased to 300; before Tarawa it was 100. Due to mechanical reliability problems after every landing, the Marines replaced all LVTs used in operations. As a result of Tarawa experience, standardized armor kits were provided for the LVTs to be used in contested landings. Other improvements were made in the damage repair area, machine gun shields, and in the LVT design to increase crew and LVT survivability. The gun-armed "amtank" LVT(A)-1 and LVT(A)-4 were developed to provide fire support. Armed with a 75mm howitzer, the latter was introduced in 1944 just before the Marianas campaign and was especially effective in this role as it was capable of destroying Japanese fortifications as it came ashore. Its howitzer complemented the 75mm gun of the M4 Sherman tanks used by the Marines. However, the LVT(A)-4 had an open-topped turret which left the crew vulnerable to artillery and infantry attack, especially to the latter, as it lacked any sort of machine gun armament. The lack of machine gun armament was eventually rectified, though the open-topped turret remained in order to save weight. Although usually used in a direct role during landings only (once inland the "amtanks" were assigned to artillery formations to augment their firepower), in the Marianas campaign "amtanks" were employed inland, much like regular tanks. Bougainville In November 1943, US Marines landed on the island of Bougainville. 29 LVTs were landed on the first day, with a total of 124 LVTs operating with the Marines during the landing. Marshall Islands In the campaign for the Marshall Islands, the full range of the LVT models became available, including armed Amtrac LVTs based on the proven LVT-2 with a tank gun turret. This provided close-in firepower as the cargo LVTs neared the beach. The combination of armoured cargo LVT-2 and the armed LVT(A)-1 together helped to capture the Marshalls far ahead of schedule. Saipan Saipan saw the massive use of the LVTs by the USMC with six battalions of cargo LVT, including the new ramped LVT-4, and two battalions of armored Amtracs, employing the new LVT(A)-4 with a 75 mm howitzer. Tinian At Tinian a field variant of the LVT-2 was created by the Seabees of the 3rd Battalion 18th Marines and 3rd Battalion 20th Marines.