The Boeing 707-321 series was the most successful aircraft in the late 50's and during the 60's until the 747 series replaced most of them Pan Am had a giant fleet of 128 B 707-x21. An nine of the 720 witch were shorter than the 707. It was the backbone of Pan Am's network and was introduced in all frequent routes Pan Am flew. In the picture above a B 707-321CF an all freighter version is shown. Pan Am had 10 of these aircraft flying for Pan Am "Clipper Cargo" around the world.