Adaptations
The Norden operated by mechanically turning the viewpoint so the target remained stationary in the display. The mechanism was designed for the low angular rate encountered at high altitudes, and thus had a relatively low range of operational speeds. The Norden could not rotate the sight fast enough for bombing at low altitude, for instance. Typically this was solved by removing the Norden completely and replacing it with simpler sighting systems.
A good example of its replacement was the refitting of the Doolittle Raiders with a simple iron sight. Designed by Capt. C. Ross Greening, the sight was mounted to the existing pilot direction indicator, allowing the bombardier to make corrections remotely, like the bombsights of an earlier era.
However, the Norden combined two functions, aiming and stabilization. While the former was not useful at low altitudes, the latter could be even more useful, especially if flying in rough air near the surface. This led James "Buck" Dozier to mount a Doolittle-like sight on top of the stabilizer in the place of the sighting head in order to attack German submarines in the Caribbean Sea. This proved extraordinarily useful, and was soon used throughout the fleet.
Read more about this topic: Norden Bombsight