It was a development of the British Course Setting Bomb Sight, or CSBS, which had been introduced in UK service in early 1918 and was demonstrated to the Navy in Washington in May 1918.
The Mark III was the standard Navy bombsight in the interwar period, from late 1918 until its complete replacement just before the opening of World War II.
The Navy was never particularly happy with it, as it lacked the accuracy to hit a ship from higher altitudes and had no way to directly account for moving targets.
The bomb aimer would then use a stopwatch to time an object's movement along that line, and use that measurement to calculate the aircraft's ground speed.
This speed and their current altitude were then entered into the bombsight, which would move a pointer fore and aft to indicate the correct spot to drop the bombs.
[1] This method of operation presented a problem for the Navy in World War I, as their primary bomb-equipped aircraft were large flying boats.
But more important to the Navy was the fact that their targets would generally be moving, and not along the wind line, meaning the bomber's approach would normally include residual sideways drift.
The bomb aimer would then tell the pilot to turn left or right until the target was moving directly along that line, and dropped at the indicated point.
[6] Looking for a solution, the Bureau of Ordnance asked gyroscope expert Carl Norden if he could come up with a stabilizing system to keep the image steady.
Against the USS Iowa steaming at 6 knots (11 km/h; 6.9 mph) directly along the wind line, they managed only 2 hits from 85 bombs from the same altitude.
[b] Norden concluded that the only way to address both of these problems was to use a system designed for stabilization from the outset and use a timing mechanism that would measure both the ground speed and that of the target using a clock.
The system had several nice features, including a gyrostabilized telescopic sight and automatic sending of angle information to the PDI.
A mechanical calculator in the bombsight then begins moving the sighting telescope in such a way that, if the estimates were perfect, objects on the ground would appear to be motionless.
As the estimates were rarely perfect, the bombardier would instead see the objects drifting in the sight, and would then adjust two controls, left-right and fore-aft, until the motion stopped.