Boeing adopted the design of the successful Model 15 as the basis for its next aircraft, a primary trainer for the Navy. It was built first as the NB-1 with air-cooled Lawrance engines, and then when outfitted with Wright-Hispano E-4 engines, as the NB-2. When the Navy tested the first Model 21, it was rejected as a trainer because it could not be put into a spin, a necessary maneuver for pilot training, and it was almost too easy to fly. After the airplane was modified to "improve" its spin characteristics, the Navy ordered 41 NB-1s and another 30 NB-2s. Five additional Model 21s were sold to Peru.