In 1924 the British Air Ministry issued specification 4/24 for a twin-engined home defence fighter, which would be employed to defend Britain against night attacks by enemy heavy bombers.

From the design proposals submitted by the manufacturers, two types were selected for prototypes development, the Bristol Bagshot and the Westland Westbury.

The Westbury was a twin-engined three-bay biplane of conventional layout, large for its day irrespective of type and remarkably so for a fighter.

The 450 hp (340 kW) Bristol Jupiter VI air-cooled radial engines were installed without cowls on nacelles that were attached on top of the lower wing, at the innermost pair of struts.

[3] Although the Westbury was judged to have good flying characteristics, and the competing Bristol Bagshot monoplane had serious structural problems,[2] it was not put into production.

However, the concept was based on the theory of no allowance sighting, which sought to install the gun at an angle at which the body lift of the projectile, due to the forward motion of the aircraft, would compensate for the effect of gravity, straightening the trajectory.

[4] Over the period of gunnery trials continued between 1927 and 1930, thirteen reports of structural damage were filed, indicating that there remained problems with absorbing the blast and recoil of the guns.

The Westbury was also used to test an Oerlikon cannon in the aft position, at angles adjustable between 40 and 60 degrees, but this gun was found to have inadequate serviceability.