In 1948, when he was a young U.S. Air Force pilot ferrying humanitarian aid in the Berlin airlift, Gail Halvorsen encountered a group of German children standing by the runway at Tempelhof Airport. As the kids peppered him with questions, he reached in his pocket and found two sticks of gum, which he broke into pieces and passed around the crowd. But it wasn’t nearly enough. Looking at the faces of all the kids who had been left out, he had a brainstorm. Tomorrow when he flew in his load of cargo, he promised the children, he would drop small...