M1 heavy tractor was a term used by the United States Army during World War II for several different models of "off the shelf" civilian tracked tractors adopted for military use. Under the Ordnance Department, these tractors were meant to tow artillery pieces, and were not equipped with blades like their Corps of Engineers counterparts. Later in the war, these tractors were replaced in the prime mover role by heavy trucks such as the Mack NO or purpose-built "high-speed tractors" such as the M4, M5, or M6. Some tractors were equipped with crane attachments for ammunition and material handling.