Google is wrong on the rainfall.  Western ND averages 14"-18" and it gets dryer as you go west.  The Billings area gets around 12"-15" annual.  I'm not from the area, but I believe that it is too dry to continuously crop using tillage.  My understanding is that it is also too dry for notill continuous small grains, but farmers are having some success with with a notill rotation using peas and small grains.