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 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.

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.