In 1963, the United States Air Force began to look for a very big plane to transport things. At that time, they were using the Lockheed C-141 Starlifter. However, the Air Force thought that a much bigger plane which could carry more cargo was needed. The Air Force called the big plane the CX-Heavy Logistics System (CX-HLS). It needed to be able to carry of cargo and travel at Mach 0.75 (). It also had to be able to fly for while carrying of cargo. The cargo bay had to be wide, high and long. There had to be doors for the bay at the front and the back of the plane.