The XM-104 was a vehicle meant to be an amphibious, airborne, and heliborne self propelled artillery peice that was meant to fufil the role of "heavyweight puncher with featherweight mobility" as said by Brigadier General Thorlin. Armed with a 105mm Howitzer and getting a combat weogjt of 3.2 Tonnes (most of the weight coming from the gun) the XM-104 is the closest thing you'll find to a true glass cannon. As told by the designation "XM" the vehicle never entered true service.