Contrary to conventional wisdom, Carl L. Norden -- inventor of the classified Norden bombsight used in World War I! -- did not invent the only U.S. bombsight of the war. He invented one of two major bombsights used, and his was not the first one in combat.

That honor belongs to the top secret product of an engineering team at Sperry Gyroscope  Co., Brooklyn,   N. Y. The Sperry bombsight out did the Norden in speed, simplicity of operation, and eventual technological significance. It was the first bombsight built with all-electronic servo systems, so it responded faster than the Norden's electromechanical controls. It was much simpler to learn to master than the Norden bombsight and in the hands of a relatively inexperienced bombardier its targeting was at least as accurate. And the autopilot that made it work so effectively became the basis for decades of commercial and military aircraft.

by Loyd Searle 

Yet although the U.S. Government authorized Sperry to construct a 186,OOO-square-meter plant in Great Neck on New York's Long Island to manufacture the bombsight and autopilot, the Army canceled the Sperry contracts less than a year after the plant's opening and handed the business to Norden and other companies. Furthermore, declassified documents, plus recollections from some of the principals, show that the design of the final Norden bombsight‹for which a patent was applied for in 1930 but not issued until 1947 incorporated many of the central improvements pioneered by engineers at Sperry.

How were the Norden and Sperry bombsights invented, and how did they compare? If both bombsights were classified, why did the Norden become so famous during World War II that it was even featured in popular movies while the Sperry was comparatively little known? What factors caused the Army's sudden reversal, even with the Sperry device's advantages? Recent synthesis from scattered documents and interviews with some of the surviving principals lend some insight into these questions.

The precision-bombing problem
Before the Norden and Sperry bombsights, accurate high altitude bombing was considered impossible. Strategists thought of bombers as unstable artillery gun platforms. In the 1930s, comparatively simple mechanisms guaranteed fair accuracy in hitting targets from altitudes below 5000 feet (1.5 kilometers). But at heights above the effective range of antiaircraft guns, aircraft moved too fast for normal calculations of firing data.

The problem of calculating in real time the proper point for releasing a bomb was formidable for the equipment then in use. A bomber traveled rapidly in three dimensions and rotated about three axes, and was often buffeted by air turbulence. The path of the dropped bomb was a function of the acceleration of gravity and the speed of the plane, modified by altitude wind direction, and the ballistics of the specific bomb. The bombardier's problem was not simply an airborne version of the artillery-gunner's challenge of hitting a moving target; it involved aiming a moving gun with the equivalent of a variable powder charge aboard a platform evading gunfire from enemy fighters.   

Originally, bombing missions were concluded by bombardier-pilot teams using pilot-director indicator (PDI) signals. While tracking the target, the bombardier would press buttons that moved a needle on the plane's control panel, instructing the pilot to  turn left or right as needed. The pilot had to maintain straight and level flight at the precise altitude and airspeed the bombardier had predetermined for the mission. If the pilot allowed those factors to vary, it would upset the bombardier's efforts to track the target; similarly, if the bombardier operated the azimuth tracking of the bombsight unsteadily, the wavering PDI signals would cause the pilot to fly the plane inaccurately.