MAXWELL AIR FORCE BASE, Ala. — Six years ago it seemed a zany idea when Lieutenant Colonel Jenns Robertson , 45, a bespectacled Minnesota native with an outsized gee-whiz quality even by military standards, began a rather unusual hobby: documenting a century of US air power — bomb by bomb.

Robertson asked: “What if you had the detailed data on when and where every bomb was dropped from an airplane in combat? What would you know?”