"... in order to drop a bomb so that it will strike at least in the vicinity desired, the use of a bomb sight is imperative. However, this sight must be simple enough ... to use it even under hostile fire."
- American Expeditionary Force Booklet on High Altitude Bombsights, Aug. 20, 1918
During World War I, the U.S. Army Air Service used bombsights provided by the Allies. The British-designed Wimperis had the reputation of only being "better than nothing at all." The French Michelin could not compensate for wind -- forcing the pilot to fly directly into or against the wind, which made the bomber an easy target for anti-aircraft gunners. Unless they flew very low over the target, a very dangerous thing to do, the bombardiers achieved only marginal results with the small bombs their airplanes could carry. Therefore, developing a high-altitude bombsight capable of correcting for crosswinds became an important project for the Army Air Service after the war.
Hitting a target with a bomb from a fast-moving airplane is a difficult task. Even if traveling at 100 mph and only a few thousand feet above the ground, a bomb dropped just half a second too late could miss its target by hundreds of feet, and that error would be amplified by flying higher to avoid ground fire. In addition, the bombardier would have trouble seeing a small target tens of thousands of feet below, but simply using a telescope with crosshairs as a bombsight would not be enough. Any turbulence would bounce the airplane and throw off the aiming, and the bombsight needed internal stabilizers to counter the movement of the airplane.
A bombardier had to use a complex formula that incorporated the trajectories of different size bombs with the effects of altitude, airspeed, true ground speed, cross winds and other variables such as hitting a moving battleship. Because doing all these computations in combat would be almost impossible, the Army Air Service needed a bombsight that could automatically perform all these calculations for the bombardier. In addition, the bombardier needed a simple method of transmitting minor course corrections to the pilot on the final approach to the target.
The Army Air Service Engineering Division at McCook Field, Ohio, undertook the development of a new bombsight after WWI. Starting with the wartime Wimperis, designated the Mark I, the engineers incorporated a series of modifications that produced limited but inadequate improvements. Due to a lack of suitable Army bombsights, Gen. Billy Mitchell borrowed U.S. Navy Mark III-A bombsights to sink the Ostfriesland in 1921.
The Engineering Division produced the D-1 bombsight in 1921. Based upon a sight designed by Georges Estoppey, the D-1 used a stopwatch to synchronize the aircraft speed with the true ground speed and used a pendulum for stabilization. In 1926 the Army adopted a later model of the Estoppey bombsight, the D-4. More heavily constructed than the D-1, the D-4 also incorporated an improved internal timing mechanism. When in perfect repair and under ideal conditions, the D-4 bombsight achieved good results up to 8,000 feet, but at higher altitudes, bombing errors became excessive.
Meanwhile, the Engineering Division worked with the Sperry Gyroscope Co. to develop a bombsight designed by Alexander P. Seversky. Completed in 1924, the Sperry C-1 bombsight used a gyroscopic stabilizer.
Another bombsight developed by the Army Air Service and built by Sperry in 1928, the Inglis L-1, synchronized the airspeed, altitude, true ground speed, and the bomb's ballistics. It also had a gyroscopic stabilizer and an automatic bomb release. The addition of pendulums or gyroscopes helped stabilize bombsights, but none of these bombsights achieved the accuracy Army aviators expected. Ironically, the solution came from merging a very accurate bombsight developed by Carl Norden for the Navy with an automatic pilot developed by Sperry for the Army. This arrangement gave the bombardier control over the aircraft during the final approach to the target. Although not as accurate as the Norden bombsight, the interwar bombsights proved superior to those used in WWI, and they gave planners the tools needed to develop the doctrine of high-altitude, daylight precision bombing adopted by the U.S. Army Air Corps in the 1930s.
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