In 1963, the United States Air Force started a series of study projects on a very large strategic transport aircraft. Although the C-141 Starlifter was being introduced, officials believed that a much larger and more capable aircraft was needed, especially to carry cargo that would not fit in any existing aircraft. These studies led to initial requirements for the CX-Heavy Logistics System (CX-HLS) in March 1964 for an aircraft with a load capacity of and a speed of Mach 0.75 (), and an unrefueled range of with a payload of . The payload bay had to be wide by high and long with access through doors at the front and rear.Norton 2003, pp. 5–12.
In 1963, the United States Air Force started a series of study projects on a very large stra?tegic transport aircraft. Although the C-141 Starlifter was being introduced, officials believed that a much larger and more capable aircraft was needed, especially to carry cargo that would not fit in any existing aircraft. These studies led to initial requirements for the CX-Heavy Logistics System (CX-HLS) in March 1964 for an aircraft with a load capacity of and a speed of Mach 0.75 (), and an unrefueled range of with a payload of . The payload bay had to be wide by high and long with access through doors at the front and rear.Norton 2003, pp. 5–12.