The tractor was motivated by the US Army Air Forces need for rescue capability in northern areas where reaching a downed crew might entail cross-country travel in snow. The M7 may be used for either highway or snow travel as the front axle can be equipped with either wheels or skis. Because of its relatively low center of gravity, the M7 tractor can easily climb steep slopes, depending on the traction available, and the load being pulled.
Year-by-year breakdown: https://en.wikipedia.org/wiki/M7_Snow_Tractor
More pics: http://mapleleafup.net/forums/showthread.php?t=5587