There are some big tanker aircraft flying around the world, including those based on the DC-10, 767 and Russia's IL-76 design. Yet the largest aerial refueler of them all is based on the iconic Boeing 747, a symbol of American might and ingenuity. Ironically, it flies for none other than the Islamic Republic of Iran Air Force.
The 747 heavy tanker-transport concept evolved out of a mid-1960s USAF initiative to field a much larger tanker than the plentiful Boeing 707 derived KC-135 Stratotanker. Eventually this movement would result in the Advanced Tanker-Cargo Aircraft program, otherwise known as the ATCA.