The novelty of the design became apparent early in 1939: the Delta 9 was to be a tri-motor monoplane with a span of over 100 ft (30 m) with a thick and wide chord centre section, outboard of which the wing was thicker and much greater in chord, in part forming one of a pair of tail booms that carried the double finned empennage.

The Delta 9 was seen as a realistic approximation to a true flying wing, with its advantage of a well-distributed load because of the absence of parts like a fuselage which did not contribute to lift.

[1] Such an unusual design called for a lot of preparatory wind tunnel work, carried out in the UK at the National Physical Laboratory, the City & Guilds, Farnborough and Queen Mary College, London.

These results encouraged the company to build the Willoughby Delta 8 to investigate the general aerodynamics of the layout with a smaller aeroplane.

[1] A twin-engined aircraft constructed of wood, the Delta 8 looked at first sight rather like a conventional pod and twin boom machine.

The tailplane joined the rearmost inner edges of the side wings, carrying the tailwheel at its centre, and a broad elevator hinged clear of the rest of the structure.

[1] The Delta 9 as described in Flight was expected to carry 36 passengers in two side wing cabins for a gross weight of 38,000 lb on three 1,000 hp engines.