The aircraft's design is attributed to John Douglas Rennie and a prototype went airborne for the first time on March 26th, 1940. However, that April, extreme aileron flutter forced the crew to bail out (three were killed in the action) and the aircraft was a ruled a total loss. Just a handful of test flights were completed with this vehicle, meant to test the validity of the retracting hull idea, and, apparently, RAF authorities had seen enough of the project to end its commitment to the B.20 after the final flight (as such no additional development work was taken up).
As designed, the B.20 carried 2 x Rolls-Royce Vulture X-type, 24-cylinder engines developing 1,720 horsepower each and these were used to drive three-bladed propellers in traditional "puller" fashion. Performance included a maximum speed of 305 miles per hour and a range out to 1,500 miles. Dimensions were a length of 69.7 feet, a wingspan of 82 feet and a height of 25 feet. MTOW was rated at 35,000lb. Armament was to be defensive in nature and made up of a mix of cannon and machine guns to include two powered turrets. The operating crew would have been six men.
In the end, a submission from Saunders-Roe, the A.36 "Lerwick", became the more successful of the two offerings but even this design was not an outright success and only twenty-one examples were acquired by the RAF (and some used by the Royal Canadian Air Force (RCAF)). The series gave a poor showing in practical wartime service as they were highly prone to accidents (indeed ten of the lot were lost in this fashion). With service introduction had in 1940, the line was retired as soon as 1942.
The Blackburn B.40 was drawn up as an improved, dimensionally larger, form of the B.20 and this was covered by Specification R.13/40 of 1940. In September of 1941, the RAF commissioned for a pair of B.40 prototypes to be developed but, after review, the type was not deemed worthy of additional work nor funding, leading to its uneventful end in December of that year. This aircraft was to carry 2 x Bristol Centaurus air-cooled radial piston engines and carry a mix of cannon and machine gun armament for defense. Dimensions included a wingspan of 98 feet and an MTOW of 52,000lb. The Short Sunderland ultimately took its place in service (detailed elsewhere on this site).
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