Rules of a competition released in July 1919 emphasised safety in terms of a short takeoff and slow landing speed as well as useful load and economy, and Bristol decided that a modification of the Tourer was their best hope.
The single passenger was enclosed in a cabin immediately behind the pilot's open cockpit, with a raised roof and windows in the decking where the second seat in the Tourer had been. The fuselage was deepened by dropping the lower longerons and floor to the lower wing spar. Ahead of the pilot, the fuselage bays were built from steel rather than wood spars. The single-axle main undercarriage carried wheels with disc brakes; there was a central skid to prevent nosing over and fenders under the wing tips. The tail skid was steerable and sprung.
The Seely was a three-bay biplane with greater wing area than the Tourer, with ailerons on both upper and lower wings. The rudder was horn balanced and the fin area generous. For the competition, it was powered by a water-cooled upright in-line 240 hp (180 kW) Siddeley Puma with a large nose radiator behind the wooden two-blade propeller.
There were only two other aircraft in the competition, held at RAF Martlesham Heath in August 1920, the Westland Limousine and the Sopwith Antelope.
Westland was the winner of the competition and Bristol retained the Seely on general duties until 1923, when it was converted into a test bed for Jupiter development, being purchased by the Air Ministry for use with the Royal Aircraft Establishment. It was fitted with a 435 hp (324 kW) Jupiter III nine-cylinder radial engine driving a steel two-bladed Leitner-Watts propeller. The Jupiter had an exhaust-driven supercharger to enhance high-altitude performance, raising the Seely's service ceiling from 18,000 ft (5,490 m) with the Puma to 24,000 ft (8,230 m). At these altitudes, the enclosed cabin provided the observer with relief from the elements.
Engine: Siddeley Puma, 240 hp (180 kW)
Gross weight: 3,000 lb (1,360 kg)
Empty weight: 908 kg / 2002 lb
Wingspan: 14.40 m / 47 ft 3 in
Length: 8.99 m / 29 ft 6 in
Height: 3.66 m / 12 ft 0 in
Wing area: 52.58 sq.m / 565.97 sq ft
Max. Speed at sea level: 177 km/h / 110 mph
Ceiling: 5486 m / 18000 ft