Did the C17 come from the C5A concept which was much older?
From the reading I have done, the C-17 was intended to address a number of shortcomings in the evolving USAF air transport fleet:
- Retirement of the C-141 fleet
- Limited range and cargo capacity of the C-130 fleet.
- Inability of the C-5 to operate from expeditionary airfields.
The C-17 exploited the lessons learned from the cancelled YC-14 and YC-15 airlifter development programs:
https://en.wikipedia.org/wiki/McDonnell_Douglas_YC-15?wprov=sfti1
https://en.wikipedia.org/wiki/McDonnell_Douglas_YC-15?wprov=sfti1
The C-17 is described in the Wikipedia article as a much larger successor to the YC-15 design.