As a contender in the 1937 US Navy shipboard single-seat fighter competition, the Seversky-designated NF-1 (Naval Fighter No 1) was derived from the basic P-35 design and flown for the first time in June 1937. Powered by a Wright R-1820-22 Cyclone nine-cylinder radial rated at 950hp for take-off and having provision for one 7.62mm and one 12.7mm gun in the forward fuselage, the NF-1 was delivered to Anacostia NAS for evaluation under the US Navy designation XFN-1 on 24 September 1937. This designation was actually applied for "book-keeping purposes", no Navy contract being issued. The NF-1 had initially flown with a vertical windscreen similar to that first fitted to the AP-1, but this had been replaced by a more conventional windscreen prior to delivery to Anacostia, and at an early phase in the evaluation the fairings attached to the main oleo legs, which fully enclosed the undercarriage when retracted, were removed. By consensus, the XFN-1 lacked the low-speed handling characteristics demanded for shipboard operation and the fighter was rejected by the Navy, further development being discontinued.