"Development of the M110 self-propelled howitzer began in mid-1950s. The first prototype, known as the T-236, was built in 1959. The M110 entered service with the US Army and US Marine Corps in 1961. It was produced for a long period of time and was exported to a number of countries. The M110 was the most powerful self-propelled howitzer in service with the US Army. This artillery system saw action during Vietnam War..."
Source: http://www.military-today.com/artillery/m110.htm