On august 16 I was flying from chicago illinois to portland oregon and about 50 minutes before landing I saw a black object outside the window. it was all black with no visible canopy, and a looked similar to the x-15 spaceplane but a little bit wider and with smaller(but still noticeable) wings. It was descending VERY fast; at least 3/2 glide path, and it had a twin all black contrail. I do not know the size of the object or how far away it was, but I would guess about a mile away and 70-80ft long. It was moving very fast and moved across my field of view through the window in about 5 seconds. I do not know where we were over but it there was a desert plaine with mountians of 3 sides. by the time I thought to wake the person sitting next to me the object was out of sight.