The relative successes of the earlier Hanriot series of biplane fighters spurred the company to invest in evolved forms centered on the base design direction. One such entry into the series that began with the HD.1 of 1916 was the "HD.7", a direct development from the HD.3 that appeared during mid-1917. This prototype-only aircraft was drawn up to compete as a possible successor to the in-service SPAD S.XIII, the classic French fighter which appeared in 1917 and went on to see production reach 8,472 units before the end.
Unlike earlier twin-seat Hanriot fighter attempts, the HD.7 reverted back to the HD.1's single-seat layout and continued some of the design lines established through the HD.3. The engine-of-choice became the Hispano-Suiza 8Fb 8-cylinder, water-cooled engine outputting 300 horsepower fitted to the nose and driving a two-bladed wooden propeller. A typical over-under biplane wing arrangement was used that was staggered , single-bayed, and with parallel strut works. The mainplanes were situated well-ahead of midships. A section of the upper wing's trailing edge was cut-out to afford the pilot better upward vision. the pilot sat in an open-air cockpit aft and under the upper wing member. The fuselage, with its slab sides, tapered towards the rear to which a single-finned, rounded rudder was affixed with low-set horizontal planes - a design form not unlike that seen in the HD.3.
To fulfill its combat-minded role, the aircraft was proposed with a pairing of 7.7mm Vickers Machine Guns arranged over the nose and set to fire through the spinning propeller blades via synchronization. Beyond this, the aircraft was limited in any other role besides air-to-air combat with the occasional ground strafe.
The prototype was made ready for a first-flight in mid-1918 with the end of the war just months away (the Armistice would be signed as soon as November 1918). The primary competition for the Hanriot product became the Nieuport 29 (detailed elsewhere on this site), another single-seat biplane design powered by the same engine and armed in the same fashion. While the Hanriot HD.7 proved a sound design, offering capable handling, performance, and firepower, it was ultimately edged out by the impressive Nieuport 29 prototype - which went on to net adoption as well as a serial production contract (1,571 units were completed). Official service introduction of this mount was had in 1922 and the type, rather amazingly, flew into the 1930s.
As for the Hanriot HD.7, the sole prototype was all that was had from the venture and development on the new fighter was ultimately abandoned as the company continued to push other biplanes for the near-future.
As completed, the HD.7 had a length of 23.7 feet, a wingspan of 32 feet, and a height of 9.9 feet. Empty weight reached 2,715lb with a Maximum Take-Off Weight (MTOW) of 4,200lb. The Hispano-Suiza 8Fb engine, rated at 300 horsepower, provided the aircraft with a maximum speed of 135 miles-per-hour which, by World War 1 standards - was impressive. Range was out to 560 miles.
In comparison, the winning Nieuport 29 could reach speeds of 146 miles-per-hour, fly out to 360 miles, and attain an altitude of up to 27,885 feet.