The NB-1 Navy trainer version of the Model 21 incorporated advanced features for the time such as its air-cooled Lawrance engine, N-shaped wing struts to eliminate the wing incidence wires, divided axle undercarriage with elastic cord shock absorbers, and an unusually thick airfoil. NBs operated either as landplanes or as seaplanes, and as seaplanes, used a single main float instead of twin pontoons used on Boeing's earlier seaplane, the Model C.