The Norden bombsight was a World War II U.S. aiming device intended for use from horizontal bombers. Its effectiveness varied both with the skill of the bombardier and the conditions of use. While, under good conditions, it was a significant improvement over previous devices, it did not come close to the accuracy of more modern aiming systems with digital electronics, and, of course no combination of "dumb bombs" and bombsight could maintain the continuous correction of precision guided munitions.
Its first ancestor was the British Mark I designed, in 1918, by LCDR Harry E. Wimperis of the Royal Naval Air Service’s Imperial College of Science. It went into a steady improvement process by Carl Norden and his company, Carl Norden, Inc. In 1935, the Norden Mark XV demonstrated an average miss distance of 138 feet, in clear air, without interference from unfriendly people with guns, at 15,000 feet.
While publicity of the time spoke of the ability to put bombs into "pickle barrels", we appear to have lost the technology to grow pickling cucumbers of the size that would require such a barrel.
The Mark XV was the mainstay of daylight bombing, especially against Germany, and to a lesser extent Japan, where the high-altitude winds were much greater. It cannot be overemphasized that the best of this performance depended on clear skies, and the ability to fly a straight course for 30 seconds or more, with no interference from fighter aircraft or anti-aircraft artillery.
It was considered extremely secret, to the extent of having a special oath for its operators:
Mindful of the secret trust about to be placed in me by my Commander in Chief, the President of the United States, by whose direction I have been chosen for bombardier training... and mindful of the fact that I am to become guardian of one of my country's most priceless military assets, the American bombsight... I do here, in the presence of Almighty God, swear by the Bombardier's Code of Honor to keep inviolate the secrecy of any and all confidential information revealed to me, and further to uphold the honor and integrity of the Army Air Forces, if need be, with my life itself. — The Bombardier's Oath
Ironically, the Germans had detailed plans but had no great use for them, preferring dive bombing over horizontal bombing whenever possible.
Principle of operation
The technology was essentially that of a purpose-built electromechanical analog computer, using gyroscopes for stabilization. Prior to the bomb run, the navigator programmed the bomb's fall time, and other factors such as the best estimate of wind. He then set the crosshairs of the bombsight proper onto the target, and then activated the Norden's providing input to the autopilot. The unit would trajectory of the bomb, given crosswind, altitude, and airspeed, and then actually release it. 
To compare it to present technology, today, one can speak of the number of targets that can be accurately attacked in one sortie, or one flight of one page. In a recent test, a B-2 Spirit aircraft released 80 independent Joint Direct Attack Munition guidance kits, on Mk. 82 500 lb. bombs in one 2-second opening of the bomb doors; the crew, with 160 more bombs to deliver, disliked holding the doors open because that interfered with their stealth capability. That version of the weapon, which has improved, generally could hit within 10 feet of their target; the targets, depending on the altitude of the aircraft, could be a radius of 15 miles or so. Triple that number of GBU-39 Small Diameter Bombs could have been delivered. With standard JDAM guidance, 50% ould have landed within 10 meters of their target, and, with enhanced GPS-aided guidance, within 3 meters.
In contrast, on the Schweinfurt raid in October 1943, the Eighth Air Force sent a force of over 250 B-17s. Using Norden bombsights, only one of every 10 of their bombs landed within 500 feet of their target. Even though 60 bombers were lost, the target was not destroyed. 
- Kaufman, Randy L. (June 2004), Precision Guided Muntions: History and Lessons for the Future, vol. Master's Thesis, School of Advanced Air and SPace Studies pp. 8-9
- }U.S. Centennial of Flight Commission, Norden Bombsight
- Norden Bombsight
- Tirpak, John A. (November 2003), "Precision: The Next Generation", Air Force Magazine 86 (11)
- "Norden M-9 Bombsight", Museum of the United States Air Force