Concrete vaults at Pratt Army Air Field secured TOP SECRET Norden Bombsights used to train bombardiers in bomb sight operation and maintenance during World War II. Constructed during the field's 1942-43 buildup, they remain an important example of how the US military protected secret equipment during the war. The first two-vault building supported a squadron of twelve aircraft, and when that squadron expanded into a bomb group of four squadrons, five vaults were built and enclosed within a wood-frame building that no longer stands. Developed in the 1920's by American engineer Carl L. Norden for the US Navy, the Norden Bombsight was a computerized instrument and a key component of precision strategic bombing against Axis forces. It incorporated a gyro stabilized automatic pilot, and after modification in 1941 by the Minneapolis Honeywell Company as the Army C-1 autopilot, it enabled a bomber to fly a straight, level course so the bombardier had a steady platform during a bombing run. Also known as the Blue Ox, it could quickly calculate and correct for wind drift and, at a preset altitude, rapidly compute the proper bomb release angle for a constant speed of closure to the aiming point. This was one of the first uses of the computer in World War II. Under optimal conditions on an undisturbed run, its accuracy was excellent, though last-second changes in altitude or the bomber's angle could sharply reduce that accuracy.