Actually, I think FedEx started flying canceled checks and other financial instruments that had to be there overnight. It grew from there.
What’s interesting is the life cycle of airliners. When the utilization is 10+ hours per day, as it is for most passenger operations, fuel efficiency is critically important.
When the utilization drops for cargo operations, as it does for FedEx, then total ownership matters much more. Buying a used 727, that is too much of a gas guzzler to remain in high utilization service, and operating it on two short legs/day, is far cheaper than a new cargo airplane.
So, FedEx, like the rest of the air cargo industry, uses new cargo planes on ping haul routes where fuel matters, and old ones for short haul.
They redefined the airliner life cycle, and an entire industry has been created to convert airplanes like 727, 757, 767, and 777 from passenger to cargo service.
There are a lot of FedEx airplanes flying around with N-numbers (the original registration of the airplane) that end in “UA”. Guess where they got those jets?