The ferry range of an aircraft is the maximum distance that the aircraft can travel without stopping when configured solely for maximum range. This typically means no (or minimal) combat stores aboard, and extra fuel when possible in the form of drop tanks or other special accomodation. It may involve the removal or internal carriage of some of the airplane's standard equipment. If the aircraft is capable of air-to-air refueling, this range is sometimes listed as 'unlimited' or dependent on pilot endurance or other maintenance factors. The term comes from the 'ferry' mission, i.e. to transport airplanes from one place to another one time for their use at the destination. This practice came into widespread use during World War II, when the United States found itself sending large numbers of combat aircraft to Europe and the Pacific Theater from the continental U.S. Some ferry missions had the aircraft's crews flying their own machines to the front lines; others (reinforcements, replacement aircraft) used flight crews dedicated to the ferry task. Although women pilots were not permitted to fly combat, they were invaluable for their efforts as ferry pilots.