It was already realized that one major source of error in bombing was leveling the aircraft enough so the bombsight pointed straight down. Even small errors in leveling could produce dramatic errors in bombing, so the Navy began a series of developments to add a gyroscopic stabilizer to various bomb sight designs. This led to orders for such designs from Estoppey, Inglis (working with Sperry) and Seversky. Norden was asked to provide an external stabilizer for the Navy's existing Mark III designs.