The W.11 "Air Horse" heavy lift helicopter was developed by the G & J Weir, Ltd., Aircraft Department, reconstituted in 1943 as the Cierva Autogiro Company.

The blades were constructed from resin-impregnated wood which provided enormous strength, and were manufactured by the Glasgow furniture firm H. Morris & Co., Ltd.[3] The W.11 rotor control system was hydraulically powered.

The landing gear had a stroke of 5 ft (1.5 m) to cater for high descent rates in the event of engine failure during low-altitude operations.

In September 1945 the design was modified to meet a requirement from Pest Control, Ltd., for use as a crop sprayer ("Spraying Mantis") in Africa for the groundnut scheme.

With a payload of 6,720 lb (3,050 kg) it would have been a very capable sprayer and following the first flights in December 1948 a grant was received from the Colonial Office to assist in development.

The death of three long-time colleagues in the accident prompted financier James G. Weir to decline to provide additional funds since the Cierva Autogiro Company required ever-increasing investment.

Development of the W.11 continued for a short time thereafter but was terminated by the British Government, and the remaining airframe, which had flown for less than 20 hours in total, was scrapped.