Development
The M113 was developed by Food Machinery Corp. (FMC), which had produced the earlier M59 and M75 Armored personnel carriers. The M113 bore a very strong resemblance to both of these earlier vehicles. The M75 was too heavy and expensive to be useful; its weight prevented amphibious capability, and being transported by air. The lightened M59 addressed both of these problems, but ended up with too little armor, and was unreliable as a result of efforts to reduce its cost.
The Army was looking for a vehicle that combined the best features of both designs, the "Airborne Armored Multi-Purpose Vehicle Family" (AAM-PVF). of all-purpose, all-terrain armored fighting vehicles FMC had been working with Kaiser Aluminum and Chemical Co. in the late 1950s to develop a suitable aluminum armor. Use of this armor could produce a vehicle that provided the protection of the M75, and the light weight and mobility of the M59.
FMC responded with two proposals; two versions of the aluminum T113 - a thicker and a thinner armored one - along with the similar but mostly steel T117. The thicker-armored version of the T113, effectively the prototype of the M113, was chosen because it weighed less than the steel competitor, while offering the same level of protection. An improved T113 design, the T113E1, was adopted by the U.S. Army in 1960 as the M113. A diesel prototype T113E2 was put into production in 1964 as the M113A1, and quickly supplanted the gasoline-engined M113. In 1994, FMC transferred the M113's production over to its newly formed defense subsidiary, United Defense. Then in 2005, United Defense was acquired by BAE Systems.
The M113 was developed to provide a survivable and reliable light tracked vehicle able to be air-lifted and air-dropped, by C-130 and C-141 transport planes. The original concept was that the vehicle would be used solely for transportation, bringing the troops forward under armor and then having them dismount for combat; the M113 would then retreat to the rear. Entering service with the U.S. Army in 1960, the M113 required only two crewmen, a driver and a commander, and carried 11 passengers inside the vehicle. Its main armament was a single .50-caliber (12.7 mm) M2 Browning machine gun operated by the commander.
On 30 March 1962, the first batch of 32 M113s arrived in Vietnam, and were sent to two Army of the Republic of Vietnam (ARVN) mechanized rifle companies, each equipped with 15 of the APCs (M113s). On 11 June 1962, the two mechanized units were fielded for the first time. During the Battle of Ap Bac in January 1963, at least fourteen of the exposed .50 caliber gunners aboard the M113s were killed in action, necessitating modifications to improve crew survivability. Soon, makeshift shields formed from metal salvaged from the hull(s) of sunken ships were fitted to the carriers, which afforded better protection. But, finding that this material could be penetrated by small arms fire, subsequent shields were constructed from scrapped armored vehicles.
The ARVN 80th Ordnance Unit in South Vietnam developed the shield idea further and commenced engineering general issue gun shields for the M113. These shields became the predecessor to the standardized Armored Cavalry Assault Vehicle (or ACAV) variant and were issued to all ARVN mechanized units during the early 1960s. The ARVNs had modified the M113s to function as "amphibious light tanks" and not as battle taxis as US designers had intended. Instead of an armored personnel carrier, the ARVN used the carried infantry as extra "dismountable soldiers" in "an over-sized tank crew." These "ACAV" sets were eventually adapted to U.S. Army M113s with the arrival of the Army's conventional forces in 1965. The vehicles continued to operate in the role of a light tank and reconnaissance vehicle, and did not operate as designed in theater. Still, the M113 could carry 11 infantrymen inside, with two crewmen operating the M113.
The U.S. Army, after berating the Vietnamese for flouting battle doctrine, came out with their own ACAV version. This more or less standardized ACAV kit included shields and a circular turret for the .50-caliber M2 machine gun in the Track Commander (TC) position, two M60 machine guns with shields for the left and right rear positions, and "belly armor" - steel armor bolted from the front bottom extending 1/2 to 2/3 of the way towards the bottom rear of the M113. The two rear machine gunners could fire their weapons while standing inside the rectangular open cargo hatch. This transformed the M113 into a fighting vehicle, but the vehicle still suffered from its lightly armored configuration, having never been designed for such a role.
In order to improve the fighting ability of the mounted troops, a number of experiments were carried out in the 1960s under MICV-65 project, which aimed to develop a true "infantry fighting vehicle" rather than an "armored personnel carrier". Pacific Car and Foundry entered the steel-armored XM701, but this proved to be too slow and too heavy to be airmobile, even in the C-141. FMC entered the XM734, which was largely the ACAV M113, but whereas the M113 seated the troops facing inward on benches along the walls, the XM734 sat them facing outwards on a central bench. Four gun ports and vision blocks were added on each side to allow the seated troops to fire even while under cover. Although neither the XM701 or XM734 were deemed worthwhile to produce, FMC continued development of their version as the XM765 Advanced Infantry Fighting Vehicle (AIFV). The AIFV was sold to a number of third party-users in the 1970s, including the Netherlands, the Philippines and Belgium.
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