The Blackburn B-20 stands as one of the most unusual and technically ambitious British aircraft of the Second World War—an experimental flying boat/floatplane hybrid built to test a radical solution to the long-standing aerodynamic penalties of seaplanes.
Conceived in the mid‑1930s and flown only briefly in 1940, the B‑20 demonstrated that a retractable planing hull could dramatically improve performance, but its promise was cut short by a fatal crash early in testing.
Despite its short life, the B‑20 influenced later Blackburn concepts and remains one of the most intriguing “what‑ifs” of British aviation.
Origins and Development
The B‑20 originated from the work of Major John Douglas Rennie, Blackburn’s chief seaplane designer.
Rennie had long been preoccupied with the inherent compromises of flying boats: their deep hulls created drag in flight, their high wings and tall structures were needed to keep propellers clear of spray, and their takeoff attitude was often suboptimal.
In February 1935, Rennie filed a patent for a retractable planing surface that could extend for water operations and retract into the fuselage for flight, giving the aircraft the clean aerodynamic form of a landplane once airborne.
The Air Ministry’s Specification R.1/36 called for a small, fast general‑purpose flying boat capable of cruising at 230 mph.
Although the contract ultimately went to the Saunders‑Roe A.36 Lerwick, the Ministry was sufficiently intrigued by Blackburn’s proposal to order a single prototype—later designated V8914—to evaluate the retractable hull concept.
Construction took place at Blackburn’s new seaplane works at Dumbarton on the River Clyde.
The aircraft was completed in early 1940 after a protracted build period, likely due to the complexity of the retractable hull and the evolving engine installation.
The B‑20 first flew on 26–27 March 1940, piloted by Blackburn test pilot Harry Bailey.
Technical Description
Overall Configuration
The B‑20 was a twin‑engine, high‑wing, all‑metal monoplane with a crew of six.
Structurally, it resembled a conventional landplane more than a traditional flying boat—when the hull was retracted.
Its most distinctive feature was the extendable planing hull, a 48 ft 9 in (14.86 m) long structure mounted beneath the fuselage on four hydraulic jacks.
When lowered, it provided buoyancy and the correct takeoff attitude; when raised, it nested into a recess in the fuselage, dramatically reducing drag.
Retractable Hull Mechanism
The hull consisted of five watertight compartments, including a central section containing four fuel tanks with a total capacity of 1,172 US gallons (4,437 L).
Four hydraulic cylinders extended the hull downward and forward, giving the aircraft the ideal angle of incidence for water operations.
Access to the aircraft on the ground or water was via hatches in the hull, including a ladder beneath the bombardier’s station.
Wings and Floats
The B‑20’s wing was a one‑piece, three‑spar structure with metal skinning.
The wingtip floats retracted outward to become streamlined wingtips—an unusual arrangement that reduced drag in flight and echoed the later appearance of tip tanks on postwar aircraft.
The floats each contained four watertight compartments and folded flush with the wing undersurface when retracted.
Powerplant
The prototype was powered by two Rolls‑Royce Vulture II X‑24 liquid‑cooled engines, each producing up to 1,800 hp for takeoff.
These engines were housed in streamlined nacelles with ventral radiators and drove three‑blade constant‑speed propellers.
The choice of Vultures—complex and temperamental engines—was likely driven by the need for high power to offset the aircraft’s weight and experimental equipment.
Crew and Internal Layout
The B‑20’s interior was unusually spacious for an aircraft of its size.
From nose to tail, it included the following:
A bombardier’s compartment with optical glazing
A cockpit with side‑by‑side pilot seating
A lower compartment for navigator, observer, and flight engineer
A wardroom with two bunks
Crew quarters with four bunks
A galley and lavatory
This arrangement reflected the aircraft’s intended role as a long‑range patrol flying boat, even though the prototype carried no armament.
Planned Armament
Although unarmed in testing, production B‑20s were intended to carry:
Two .303 machine guns in the nose
A dorsal turret with two .303 guns
A tail turret with four .303 guns
Four internal bomb cells in the wing center section, each capable of carrying a 500 lb bomb
Performance
The B‑20’s performance—especially with the hull retracted—was exceptional for a flying boat of its era.
Without turrets installed, the aircraft achieved the following:
322 mph (518 km/h) at 15,000 ft
302 mph (486 km/h) at 5,750 ft
280 mph (451 km/h) at sea level
With full defensive armament, maximum speed was still an impressive 306 mph (492 km/h) at altitude.
The range was approximately 1,500 miles (2,414 km), and cruising speed was around 200 mph (322 km/h).
During one test flight, the B‑20 reportedly reached 345 mph (555 km/h)—an extraordinary figure for a seaplane in 1940 and a testament to the aerodynamic benefits of the retractable hull.
Testing and Crash
Early test flights revealed minor aileron issues but otherwise confirmed the soundness of the retractable hull concept.
Water handling was excellent, and the hull mechanism worked reliably, though not always smoothly in flight.
On 7 April 1940, during high‑speed trials over the Firth of Clyde, the aircraft suffered catastrophic aileron flutter, causing loss of control.
Of the five crew aboard, only two survived.
Test pilot Harry Bailey bailed out but was too low for his parachute to deploy fully; two Blackburn riggers were also lost.
The wreck sank in deep water and remains a protected war grave.
One of the Vulture engines was recovered in 1998 after being snagged by a trawler and is now displayed at the Dumfries and Galloway Aviation Museum.
The Air Ministry concluded that the crash was unrelated to the retractable hull design, but wartime priorities prevented further development.
Context in WWII Aviation
The B‑20 emerged at a moment when Britain urgently needed reliable maritime patrol aircraft.
The Air Ministry’s preference for the Saunders‑Roe Lerwick—and the proven Short Sunderland—reflected a desire for immediate operational capability rather than experimental innovation.
Nevertheless, the B‑20 demonstrated that flying boats could achieve landplane‑like performance if freed from the aerodynamic burden of a fixed hull.
Had the war not intervened, the concept might have matured into a new generation of high‑speed maritime aircraft.
Instead, the rapid rise of long‑range land‑based patrol bombers (e.g., Liberators and Fortresses) and the decline of flying boats after 1945 ensured that the B‑20 remained a technological dead end—brilliant, promising, but ultimately orphaned by history.
Legacy
Although the B‑20 never entered production, its retractable hull concept was validated in flight and remains one of the most innovative ideas in seaplane design.
The aircraft’s brief career left behind:
A proven aerodynamic principle
Two ambitious but unrealized derivative designs
A tragic but historically significant crash site
A recovered Vulture engine as a tangible reminder of the project
Today, the Blackburn B‑20 is remembered as a bold experiment—an aircraft decades ahead of its time whose potential was never fully explored due to the pressures and priorities of global war.
