The xc-99 was a successful program for the very same reason that the B-36 succeeded. It was the final version, excepting later failures like the XF-12 and XB-35, of large winged piston engine powered a/c.
Turbo props took over transports.
Over 7500 hours which is not bad.
But notice and pay attention people. All big military transport projects that are approved carry a civilian market promise. As was noted earlier, the C-99 was proposed as the Convair 37.
The C-141 was proposed as the SuperStarLifter.
But at a cost ratio of 5 to 10 to 1, these are usually moot points.