M109 155mm SP Howitzer

In January 1952, a conference in Washington DC on the subject of self-propelled artillery indicated an urgent need for improved self-propelled artillery. Preliminary concept studies began for a self-propelled howitzer to replace the existing 155mm M44. The first designs in August 1952 for the new vehicle, designated the Howitzer 156mm Self-Propelled T196, were rejected, as were additional studies presented in September 1953. At a conference in May 1954, a concept was finally approved.

In June 1954, a review of the military characteristics of the entire self-propelled program decided that future concepts of the T196 would be prepared along the design proposed for the Howitzer 110mm Self-Propelled T195, which was already in production. In June 1956, it was decided to use the basic hull and turret of the T195 but the original 156 mm howitzer was replaced by a 155 mm howitzer after NATO had standardized this caliber in 1956.

In October 1956, the mockup of the T196 was reviewed and verbal authority was given to proceed with development of the first prototype. The main differences from the T195 were power elevating and turret traverse mechanisms, different ammunition racks and two spades at the rear of the hull. The first prototype of the T196 was completed in 1959, about six months later than the 105 mm T195. During preliminary User Evaluation at Fort Knox a number of failures occurred in the suspension. The prototype differed from later vehicles in that it had a different shaped hull and turret, the seventh roadwheel acted as the idler and it was powered by a Continental petrol engine.

In 1959 a policy was established that diesel rather than petrol engines would be used for future combat vehicles and the prototype of the T196 was then fitted with a diesel engine and redesignated the T196E1. In February 1961, an order was placed for two T196E1 preproduction vehicles which were delivered within six months. After further trials the T196E1 was classified as a Limited Production Type in December 1961.

In October 1961, a letter order was given to the Cadillac Motor Car Division for one year's production of the T196E1 at the Cleveland Army Tank Plant. The first production vehicles were completed in October 1962. In January 1963, an extension was authorised to continue the classification of Limited Production.

In July 1963 the T196E1 was classified as standard A and designated the Howitzer, Medium, Self-Propelled: 155 mm, M109. Early in 1963, a contract was awarded to Cadillac for the second year of production. The contract for the third year of production, awarded in December 1963, went to the Chrysler Corporation, although production remained at the Cleveland Army Tank Plant. The first M109s were issued to the US Army in June 1963.