The Naval Aircraft Factory PN was a series of open cockpit American flying boats of the 1920s and 1930s. A development of the Felixstowe F5L flying boat of the First World War, variants of the PN were built for the United States Navy by Douglas, Keystone and Martin. Development and design The PN flying boats were ​twin-engine biplanes with their engines mounted in nacelles between the fabric-covered wings. Other than on the PN-11 which had a different hull form, the hull had large chines running back to the first step similar to those on the F.5L. It had a standard crew of five, but was capable of carrying a relief crew for long patrols. Early machines had wood hulls and wings, with the wings covered in fabric, while various versions replaced these with metal components. A wide variety of V-12 and radial engines were fitted, due to problems with several of the engines chosen, with later versions generally using radial engines. At the end of World War I the United States Navy was operating the Curtiss H.16 and the Felixstowe F5L Long-range patrol flying boats, which had been developed in collaboration with the British. The F.5L was a license-built version of the British Felixstowe F.5 using the American Liberty engine. The series of Felixstowe flying boats, developed by the Seaplane Experimental Station, had started with improving the hull of the Curtiss America. PN-5 was the new designation assigned to F.5Ls built by the Naval Aircraft Factory after 1922, although identical aircraft built before then retained the F.5L designation. Under the new designation system, P indicated that the role was Patrol, and N denoted aircraft built by the US Navy's Naval Aircraft Factory. Numbers 1-4 were skipped. The PN-6 was had an enlarged curved fin with an unbalanced rudder. The first two examples had been ordered as the final two F.5Ls, and had been designated as F.6Ls, with the new PN-6 designation coming into effect in 1922. Some F.5L/PN-5s were later upgraded to PN-6 standard. Neither the PN-5 or the PN-6 designation was used outside of official paperwork, and these aircraft were simply referred to as F.5Ls. The PN-7 was a 1925 development with an improved thicker section, shorter-span wings with a more modern modern airfoil section, and were powered by two experimental Wright T-2 V-12 engines. Two were built. The PN-8 was developed due to problems with the wooden hull,