The aircraft was rectangle in shape and moved very slow with no sound.As I was traveling eastbound on HWY 200, just 3 miles east of the Montana/North Dakota boarder, I looked to the southeast and saw a metalic object. The object was rectangle in shape. It was a very sunny day and the sunlight was reflecting from it greatly. The object was traveling at what appeared to be a slow rate of speed. It was heading what appeared to be in a east by south east direction.I stopped my vehicle at approximately mile marker 4.5. I shut my vehicle engine off and rolled my window down. The object did not make any sound what so ever.I have studied military aircraft all of my life and did not recognize this aircaft at all. The aircraft eventualy turned south and disappeared. End of Report.