The first prototype of this design, known to the Navy as the Mark XI, was delivered to the Navy's proving grounds in Virginia in 1924. In testing, the system proved disappointing. The circular error probable (CEP), a circle into which 50% of the bombs would fall, was wide from only altitude. This was an error of over 3.6%, somewhat worse than existing systems. Moreover, bombardiers universally complained that the device was far too hard to use. Norden worked tirelessly on the design, and by 1928 the accuracy had improved to 2% of altitude, enough that the Navy's Bureau of Ordnance placed a $348,000 contract for the devices.