After 1960, all 707-320 Intercontinentals had taller tails, and, after 1962, all incorporated improvements to the leading and trailing edges of the wing. The most significant change however, was the installation of an 11-foot-wide cargo door and stronger undercarriage in the 707-320Cs. The mix of cargo and passengers on the main deck of these converible versions could be adjusted according to need. As the cargo business grew, the Cs became the most popular version of the 707, with 337 sold.