Experimental, WWII

Blackburn B20: the Flying Boat with a Retractable Belly 

Kept secret until 1945, the Blackburn B20 was a classified experimental flying boat most renowned for its innovative lower fuselage which could be retracted or extended depending on whether it was floating or flying. Hailed as an impressive and imaginative feat of modern engineering, the B20 was ultimately cancelled following a disastrous crash over the turbulent seas off the Scottish coastline.



Following the end of World War One the development of flying boats proceeded at an astonishing pace. However, in a bid to avoid the propellor from hitting the water, by the 1930s flying boats such as the Shorts ‘C’ Class and Boeing Clipper were usually made with extremely deep fuselages, making it far from streamline.

In an attempt to correct this, later planes such as the Consolidated Catalina and the Dornier Wal were engineered with their wings and engines placed at the top of the fuselage, yet such a design still produced a lot of drag.

Thus, on the lookout for a smaller and more efficient model, the British Air Ministry issued Specification R.1/36 on March 2nd 1936 requiring the assembly of a flying boat that would have a maximum cruising speed of at least 230 miles per hour and most importantly a weight that didn’t exceed 25,000 pounds.

Do 24 on water.
The goal of keeping propellers away from the water resulted in flying boats like this, the Dornier Do 24.


Responding to the call, the chief seaplane designer of Blackburn Aircraft Company, Major J.D. Rennie, proposed a revolutionary new design that addressed many of the issues of the past. He envisioned a flying boat with a hull that could not only be extended for landing, but be retracted for optimum aerodynamic conditions whilst airborne.

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Despite the radical nature of their submission, in the end Blackburn would lose out to the Saro S36 Lerwick, which would go on to have a disappointing trajectory, with only 21 ever produced. However the competition had been close, and Air Ministry officials were so impressed by the Rennie’s concept that they authorized the construction of 2 prototypes.

Although initially planned to be propelled by two Bristol Hercules engines, this would soon change to the more powerful Rolls-Royce Vulture 24 cylinder as the projected weight of the B-20 increased.

Rolls-Royce Vulture X-24 engine.
The Rolls-Royce Vulture was a 42 litre X engine with 24 cylinders. Developed in the 1930s, it was essentially four engine blocks joined together.

Development of the first prototype was carried out over 3 years, but with the outbreak of World War Two in 1939 the second prototype was cancelled, and Blackburn were instead required to pool all their resources into the construction of preexisting flying boats needed to detect the swarms of German U-boats invading the Atlantic.

Nevertheless work still continued on the original V8914 model at Blackburn’s Clydeside factory in Dumbarton, where it was given the sobriquet ‘Nutcracker’ in reference to the rather grisly fate that would theoretically befall an individual who stayed on the lowered platform when it was being retracted.

After being finished in 1940, the B20 would next undergo a flight test program that was to be conducted in total secrecy.

The B20

The Blackburn B20 was an all metal high wing cantilever monoplane weighing 35,000 pounds that was 69 feet and 7 inches long, 11 feet and 8 inches high with its bottom retracted, and 25 feet and 2 inches with it extended. The wings, which were 1066 square feet in area had wing tip floats, which when retracted meant the B-20 had a span of 76 feet, increasing to 82 feet and 2 inches if extended.

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The B20 was ran by a pair of Rolls-Royce Vulture X liquid cooled engines mounted in nacelles protruding from the leading edge of the mainplane that powered two 3 bladed de Havilland Hydromatic variable-pitch propellors, which gave it a maximum speed of 345 miles per hour, a cruising speed of 200 miles per hour, and an operational range of 1500 miles. 

Blackburn B-20.
The bizarre B20.

Its hull had stressed skin and flush-riveted plating, while the detachable bottom fuselage, called a pontoon, was sub-divided into 5 sections that housed four fuel tanks with a maximum capacity of 976 gallons as well as an array of marine equipment including an anchor, anchor winch, boat hook, mooring pennant, and a drogue.

The B20 had space for a crew of five who entered the plane by climbing up a ladder through a hatch situated on the floor of the bomb aimer’s compartment at the extreme end of the nose. Behind this segment was a spacious flight deck where the pilot and co-pilot sat together, with the navigator, observer, and engineer stationed behind them on a lower level. 

In the interior of the hull the B20 was decked out with all the comforts necessary for longer missions, featuring an officer’s wardroom with two bunks, sleeping quarters for four crew, a storage unit for loose items, an engineers’ workshop, and even a toilet placed behind the galley.

B20 adjustable pontoon hull.
Close view of the B20’s collapsible pontoon. This enabled the engines to stay away from the water during take off and landings, but didn’t create the drag-related drawbacks of typical flying boat hulls.

Although the B20 prototype was weaponless, the production version was planned to have two .303 machine guns in the nose, another pair midship in a power driven turret, and four more installed in a tail turret. It would also be armed with four bomb cells grafted to the centre section of the mainplane and just inside each engine nacelle, which would each be able to carry a 500 pound bomb.

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On its very first flight in March 1940, the V8914 prototype was found to have problems with its aileron trims, an issue that was eventually fixed after a handful of more tests that were slightly marred by a birdstrike causing minor damage. Nevertheless despite the setbacks, the retractable pontoon mechanism was found to be reliable and effective. 

With the B20 all fixed up, on April 7th 1940, the day before Germany launched its invasion of Denmark and Norway, Blackburn’s premier flying boat dashed down the runway for its first high-speed flight evaluation with Flight Lieutenant Harry Bailey at the controls. With him were Ivan Waller, a Rolls-Royce flight test engineer, Fred Weeks, a Blackburn flight test engineer, and two other passengers called Duncan Roberts and Sam McMillan who were to monitor the instruments.

Soaring over the skies of the Forth of Clyde and the Sound of Bute on Scotland’s windswept western coast, Bailey accelerated the B20 to 345 miles per hour before an ominous vibration started to set in. Slowing down, the buzz continued to persist, compelling Bailey to order his crew to get their parachutes ready for immediate evacuation.

B20 Blackburn.
The B20 floating on water. The physical benefits of its retractable belly are obvious.

The first to bail out was Fred Weeks through the escape hatch on the roof of the cockpit, during which his leg was hit by the rudder and embedded with a metal splint. Next was Ivan Waller whose parachute became entangled in the radio mast after he released it too early.

Perilously sticking halfway out of the hatch, Ivan was able to get out and carefully walk along the fuselage when the vibration temporarily subsided, disentangle his parachute, and jump free. Weeks and Bailey would survive the ordeal, but Waller, Roberts, and McMillan would not be so fortunate after their jumps.

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Waller would ultimately drown because his parachute did not open fully, while Roberts and Macmillan, presumably succumbing to the same fate, were never seen again. Its crew having noticed a rectangular unit descending from the sky, the HMS Transylvania picked up Weeks and Waller, saving them from a similar watery end.

B20 in flight.
When in flight the B20’s belly was raised up, bringing it more in line with a conventional land-based aircraft.

Nearly 60 years later on August 28th 1998 a team of Royal Navy Divers would help recover the B20’s Rolls Royce engine after it became entangled in the net of a fishing trawler which had dragged it into shallower water. Separating it from the propellor, the engine was raised to the surface using airbags, before it was transferred to the Rolls Royce Museum in Glasgow and then later onto the Dumfries and Galloway Aviation Museum.

As of today, the B20 is still at rest in its watery grave off Gourock Head. 


Although the April tragedy had sent shockwaves through the Blackburn Aircraft Company, the brief bout of examinations beforehand had proven the merits of the design conceived by Major Rennie, who continued to push for a successor to the B20. Rennie imagined an improved version would be powered instead by Napier Sabre or Bristol Centaurus engines and would additionally possess a fully retractable dorsal turret.

Consequently in September 1941 Blackburn were awarded with Contract Actf/1474/C20b requiring the construction of two prototypes that would have serial numbers ES966 and ES979. Being faster and more advanced, the B40 was originally considered as a replacement for the older Sunderland, a lumberingly big flying boat with superfluous amounts of space.

The Blackburn on water.
Ultimately, the B20’s unusual design was not successful.

On the other hand, the project started going downhill with the release of a brand new set of specifications on March 3rd 1941, which included a revised combat range of 3500 miles that the B40 simply couldn’t meet.

Development dragged on for a couple more months before it was stopped in its tracks on December 20th, when N.E. Rowe of the Air Staff announced that the B40’s anticipated range of 3050 miles without a bomb was not acceptable.

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With the Air Ministry no longer in need of a high performance flying boat for operational use, the program was officially shelved after the Christmas break on January 6th 1942.