The Blériot XXI was an early shoulder-wing monoplane that was powered by a two-bladed propeller driven by a 52 kW (70 hp) Gnome Gamma 7-cylinder rotary engine. Both the pilot and the passenger were seated in a side-by-side configuration. The control column for the Blériot XXI was centrally mounted, and there were two sets of rudder pedals. This meant that the aircraft could be flown from either seat.

The Blériot XXI was one of the two Blériot designs entered for the 1912 British Military Aeroplane Competition.

The shallow section rectangular fuselage tapered to a horizontal knife-edge at the tail, with the elongated triangular horizontal stabilizer mounted in the middle, its covering blending into that of the upper and lower fuselage surface. A semi-elliptical elevator was mounted on the trailing edge, and a triangular balanced rudder was mounted above the rear fuselage. Lateral control was affected by wing-warping, the wires leading to a single inverted V-strut cabane above the fuselage and a similar V strut beneath. Petrol was stored in three tanks: a pair of gravity tanks were located under the top decking in front of the cockpit, pressure-fed from a larger tank under the seats.

The undercarriage was a variant of the well-proved pattern used on the Blériot XI, with the wheels mounted on a trailing arm free to slide up and down and sprung by bungee cords. Unlike that of the Blériot XI, the main vertical members terminated in a crossbar below the fuselage rather than one attached to the upper longerons, and it was also braced by a pair of diagonal struts on either side, the front pair leading to the front of the upper longerons and forming the front of the raked engine cowling. The tail skid was made of two U-shaped pieces of rattan cane.