The Curtiss HA, nicknamed the "Dunkirk Fighter," was an experimental two-seat biplane seaplane fighter developed during the final stages of World War I. Designed by Captain B. L. Smith of the United States Marine Corps, the aircraft was intended for use by the US Navy and Marine Corps. The first prototype was ordered in December 1917 and completed its maiden flight on March 21, 1918. The aircraft was characterized by a wood-frame fuselage with fabric covering and a float configuration consisting of one large central float and two smaller balancing floats at the wingtips.
Technical development of the HA was marked by significant stability issues. The original prototype was found to be highly unstable, largely due to an overly heavy tailplane, and was ultimately destroyed in a crash during the testing phase. To address these flaws, Curtiss developed two subsequent prototypes, the HA-1 and HA-2. The HA-1 utilized salvaged parts from the first prototype and featured a redesigned radiator and tailplane, with the wings shifted further aft to improve handling; however, this aircraft was later lost to an in-flight fire. The HA-2 introduced a wider wingspan and further refinements, resulting in the most successful version of the series. This variant achieved a maximum speed of 118 mph (190 km/h) and a climb rate of 790 ft/min.
Powering the HA was the Liberty 12, a water-cooled V-12 piston engine producing 360 hp (270 kW). The aircraft was heavily armed for its class, equipped with two fixed, synchronized Marlin .30-caliber machine guns firing forward and two .30-caliber Lewis guns mounted on a Scarff ring for the rear observer. Despite the performance gains seen in the HA-2, the end of World War I precluded any series production orders.
Following the cancellation of military interest, Curtiss converted three of the aircraft into land-based versions to be used as mailplanes for civil airmail service. In total, only six aircraft were ever produced: three seaplane prototypes and three landplane conversions. No examples of the Curtiss HA are known to survive today, and the type remains a historical example of the challenges associated with high-powered naval aircraft design in the late 1910s.