The Boeing NB (Model 21) was a US Navy trainer and the next Boeing military aircraft after their successful PW-9/ FB fighter. The Model 21 was designed in response to a US Navy requirement for a basic trainer. It was an equal span biplane with a wide wing centre section on both upper and lower wing and outer wing panels that could be used on either the upper or lower wing. It had N type inter-plane struts, a tail skid, divided axle main landing gear with rubber bungee shock absorbers, a tail skid and the ability to be used as a floatplane. It had tandem open cockpits and was powered by a Lawrance J-1 ail cooled radial piston engine.
The Navy tested the prototype under the designation VNB-1 (V for heavier-than-air, N for Naval Trainer, B for Boeing) and decided that it was too easy to fly and almost impossible to spin. Boeing was asked to make it easier to spin and harder to fly and a production order was placed for 41 NB-1 trainers. The first of these was delivered on 5 December 1924, and it quickly became clear that Boeing had gone too far the other way. The NB-1 could easily get into dangerous flat spins from which it was almost impossible to recover. Boeing had to redesign the NB-1 so that it was harder to spin, although the design was never entirely satisfactory.
Most of the NB-1s and NB-2s went to the training squadrons VN-1D8 and VN-438 at Pensacola, VN-6D5 at Hampton Roads and VN-7D11 at San Diego. A number were modified for crop spraying and were used by Squadron VO-6M in Puerto Rico.
The first 41 aircraft were delivered with the Lawrance J-2 or J-4 engines. Some were later given a 220hp Wright J-5 engine after Wright took over the Lawrance company. The last two were kept by Boeing for experiments to improve handling, as the NB-3 and NB-4. These efforts failed and they were returned to the Navy as NB-1s.
Some NB-1s were used for anti-mosquito spraying by the US Marine Corps, as the aircraft could carry a significant load.
Thirty NB-2s were ordered. They were produced entirely to use up some of the war-surplus 180hp Wright-Hispano E-4 engines in US Navy stocks.
The sole NB-3 was NB-1 no.40 with a longer fuselage (16in at the nose, 25in at the tail), a modified tail and a Wright-Hispano E-4 engine. The NB-3 was tested in mid-1925, but was no better than the NB-1. It was then returned to NB-1 standard and delivered to the Navy.
The NB-4 was NB-1 no.41. It has the same changes as the NB-3, but with a lighter Lawrance engine, which was moved 26in forward. Testig began on 9 July 1925, but once again there were no improvements so it was returned to the Navy as an NB-1.
Boeing built five Model 21s for export to Peru, three in 1924 and two three years later in 1927.
Engine: Lawrance J-1 radial piston engine
Span: 36ft 10in
Length: 28ft 9in (as floatplane)
Height: 11ft 8in
Empty Weight: 2,136lb
Maximum Take-off Weight: 2,837lb
Maximum Speed: 99mph
Cruising Speed: 90mph
Range: 200 miles
Guns: One flexibly mounted 0.3in machine gun in rear cockpit