Boeing FB-4

The Boeing FB-4 was an experimental version of the U.S. Navy's FB series of biplane fighters, themselves the naval counterpart to the Army's PW-9. Delivered in early 1926 as the fourteenth aircraft of the original Navy order, the FB-4 was used to test a 450 hp Wright P-1 radial engine in place of the inline Packard powerplant of its siblings. It could be fitted with twin floats and had provisions for hoisting aboard ship. Trials with the FB-4 helped demonstrate that air-cooled radial engines were better suited to carrier operations than heavier liquid-cooled inlines, an important lesson for naval aviation, although the particular Wright engine proved unsatisfactory.

The single FB-4 was subsequently re-engined with a Pratt & Whitney Wasp and redesignated FB-6. The FB family equipped some of the Navy's earliest carrier fighter units in the mid-1920s.