The Norden Mk. XV, known as the Norden M series in US Army service, is a bombsight that was used by the United States Army Air Forces (USAAF) and the United States Navy during World War II, and the United States Air Force in the Korean and the Vietnam Wars. It was an early tachometric design, which combined optics, a mechanical computer, and an autopilot for the first time to not merely identify a target but fly the airplane to it. The bombsight directly measured the aircraft's ground speed and direction, which older types could only estimate with lengthy manual procedures. The Norden further improved on older designs by using an analog computer that continuously recalculated the bomb's impact point based on changing flight conditions, and an autopilot that reacted quickly and accurately to changes in the wind or other effects. Together, these features promised unprecedented accuracy for daytime bombing from high altitudes. During prewar testing the Norden demonstrated a 150-foot (46 m) circular error probable (CEP), an astonishing performance for that period. This precision would enable direct attacks on ships, factories, and other point targets. Both the Navy and the USAAF saw it as a means to conduct successful high-altitude bombing. For example, an invasion fleet could be destroyed long before it could reach US shores. To protect these advantages, the Norden was granted the utmost secrecy well into the war, and was part of a production effort on a similar scale to the Manhattan Project: the overall cost (both R&D and production) was $1.1 billion (equivalent to $15 billion in 2024), as much as 2/3 of the latter or over a quarter of the production cost of all B-17 bombers. The Norden was not as secret as believed; both the British SABS and German Lotfernrohr 7 worked on similar principles, and details of the Norden had been passed to Germany even before the war started. Under combat conditions the Norden did not achieve its expected precision, yielding an average CEP in 1943 of 1,200 feet (370 m), similar to other Allied and German results. Both the Navy and Air Forces had to give up using pinpoint attacks. The Navy turned to dive bombing and skip bombing to attack ships, while the Air Forces developed the lead bomber procedure to improve accuracy, and adopted area bombing techniques for ever-larger groups of aircraft. Nevertheless, the Norden's reputation as a pin-point device endured, due in no small part to Norden's own advertising of the device after secrecy was reduced late in the war. The Norden saw reduced use in the post–World War II period after radar-based targeting was introduced, but the need for accurate daytime attacks kept it in service, especially during the Korean War. The last combat use of the Norden was in the US Navy's VO-67 squadron, which used it to drop sensors onto the Ho Chi Minh Trail in 1967. The Norden remains one of the best-known bombsights.
History and development
Early work The Norden sight was designed by Carl Norden, a Dutch engineer educated in Switzerland who immigrated to the US in 1904. In 1911, Norden joined Sperry Gyroscope to work on ship gyrostabilizers, and then moved to work directly for the US Navy as a consultant. At the Navy, Norden worked on a catapult system for a proposed flying bomb that was never fully developed, but this work introduced various Navy personnel to Norden's expertise with gyro stabilization. World War I bomb sight designs had improved rapidly, with the ultimate development being the Course Setting Bomb Sight, or CSBS. This was essentially a large mechanical calculator that directly represented the wind triangle using three long pieces of metal in a triangular arrangement. The hypotenuse of the triangle was the line the aircraft needed to fly along in order to arrive over the target in the presence of wind, which, before the CSBS, was an intractable problem. Almost all air forces adopted some variation of the CSBS as their standard inter-war bomb sight, including the US Navy, who used a modified version known as the Mark III. It was already realized that one major source of error in bombing was levelling the aircraft enough so the bombsight pointed straight down, even small errors in levelling could produce dramatic errors in accuracy. The US Army did not adopt the CSBS and instead used a simpler design, the Estoppey D-series, as it automatically levelled the sight during use. Navy experiments showed these roughly doubled accuracy, so they began a series of developments to add a gyroscopic stabilizer to their bombsights. In addition to new designs like the Inglis (working with Sperry) and Seversky, the Navy also asked Norden to provide an external stabilizer for the existing Mark III designs.
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