The Boeing NB (or Model 21) was a primary training aircraft developed for the United States Navy in 1923. It was a two-bay, equal-span biplane of conventional configuration with interchangeable wheeled and float undercarriage. The pilot and instructor sat in tandem, open cockpits.
The NBs were produced in two batches; the first (NB-1) were powered by radial engines and the second by war-surplus V-8s still in the Navy's inventory. The original prototype evaluated by the Navy had been assessed as being too easy to fly, and therefore of limited use as a trainer. In particular, it was noted that the aircraft was impossible to spin. The NB-1 design attempted to introduce some instability, but it was soon discovered that while it was now possible to get the aircraft into a spin, it was virtually impossible to recover from one. A series of modifications were made to attempt a compromise.
Specifications (NB-1).
General characteristics* Crew: two, pilot and instructor* Length: ()* Wingspan: ()* Height: ()* Wing area: ()* Empty weight: ()* Gross weight: ()* Powerplant: 1 × Lawrance J-1 
 , 200 hp (149 kW) 
Performance* Maximum speed: (, ) * Range: (, ) * Service ceiling: ()* Rate of climb: ()
Armament