Though the Nie 31 was technically a sesquiplane, it could equally well be described as a shoulder-wing monoplane with a small foreplane. Its immediate predecessor was the Nieuport Madon, which had an ancillary lifting surface built around the axle of its wide-track undercarriage. The Nie 31 had a narrower-track undercarriage, but the second wing extended outwards beyond the wheels, which were faired into its upper surface; this wing and the axle were joined to the lowerfuselage by a pair of near vertical N-form struts. The main plane was of low aspect ratio. In plan it had a rounded leading edge, straight trailing edge and square tips. Its roots were cut away to allow a better downward view from the cockpit. This was placed ahead of the trailing edge so the pilot looked forward over the wing; he had a faired headrest behind. The Nie 31's upper wing had ailerons but no flaps and was braced on each side by a parallel pair of aerofoilsection struts stretching outwards from the bottom of the undercarriage struts. The lower wing had parallel chord and straight edges, providing a lifting surface with about 40% the area of the main plane. Both wings were fabric covered. The Nie 31 was powered by a Le Rhone 9R nine-cylinder rotary engine, smoothly and completely cowled. It drove a two-blade propeller. The fuselage, like that of the earlier Nieuport Nie 29, was a smooth wooden monocoque shell of tulipwood, produced by winding thin spiral strips around a mould in a series of alternately handed layers and glueing them together. It tapered to the rear, where an almost delta-shaped tailplane was mounted at mid-height, carrying horn-balancedelevators. The fin was much less broad, with a straight-edged, balanced rudder extending down between the elevators below the lower fuselage, where there was a small dorsal fin. The Nie 31 was intended to carry twin 7.7 mmVickers machine guns. A flight test programme conducted through 1919 showed that the Nie 31 had excellent performance, especially given the low engine power, but Nieuport decided not to proceed with its development.
