Originally, the Bristol Blenheim was not created as a bomber or with
the RAF in mind. During 1933 Bristol Chief Designer Frank Barnwell
announced a proposal for a high-speed light passenger aircraft, the
Bristol Type 135. The Type 135 as envisioned as a low-wing monoplane
capable of carrying up to eight passengers within an all metal
cantilever stressed skin fuselage, powered by two 500 hp (373 kW) nine
cylinder Bristol Aquila I sleeve-valved air cooled radial engines. By
1934 work on the design had advanced to the fuselage mock-up stage and
it was decided to display the mock up at the 1935 Salon
Internationale de L'Aeronautique in Paris.
1934 Lord Rothermere, who was the owner of the Daily Mail newspaper,
expressed a desire to obtain for his personal use, a fast and spacious
private aeroplane, for this aviation-minded organisation had then
appreciated the potential of what is today called the business or
corporate aircraft. Lord Rothermere envisaged his requirements as a
fast aircraft that would accommodate a crew of two and six passengers,
and it just so happened that the Bristol Aeroplane Company had already
drawn up the outline of a light transport in this category, the Type
new aircraft had been designed originally to be powered by two 500 hp
(373 kW) Bristol Aquila I engines which were then under development.
The Type 135 had an anticipated top speed of 180 mph (290 km/h) but
lacked the range to meet Lord Rothermere's requirements. Frank Barnwell
proposed changes that included reducing the fuselage cross section to
reduce drag and replacing the 500 hp (373 kW) Bristol Aquila I engines
with a couple of 640 hp (477 kW) Bristol Mercury VI radial engines
driving fixed pitch four blade propellers. Design work started on the
now designated Bristol Type 142 with Lord Rothermere as its principle
source of funding. It would cost him £18,500 to complete the aircraft,
a large sum even by todays standards. Bristol had learned of government
plans to expand the RAF and with the anticipation of possible future
contracts decided to fund a parallel design called the Bristol Type 143
as a private venture. The Type 143 featured a longer nose and longer
First flown at Filton on 12 April 1935, the Type 142 was to spark off
much comment and excitement when during its initial trials it was found
to be some 30 mph (48 km/h) faster than the prototype of Britain's
most-recently procured new biplane fighter, the Gloster Gauntlet. Named
Britain First, it was presented to the nation by Lord Rothermere
after the Air Ministry had requested that they might retain it for a
period of testing to evaiuate its potential as a light bomber. It had a
number change from G-ABCZ to K-7557 and was then moved to Martlesham
Heath for RAF trials. It proved so successful that in 1935 the Air
Ministry issued Specification B.28/35 for a military version with
similar performance. This, then was the sire of the Bristol Blenheim
which was to prove an important interim weapon at the beginning of
World War II.
The Bristol Mk IV dorsal turret on a Blenheim Mk IV aircraft.
Aware of Air Ministry interest in the Type 142, Bristol busied
themselves with homework to evolve a military version (Type 142M) of
this aircraft, and in the summer of 1935 the Air Ministry decided to
accept the company's proposal, placing a first order for 150 aircraft
to Specification B.28/35 in September. The new aircraft was very
similar to the Type 142, but there had of course been some changes to
make it suitable for the military role, primarily to accommodate a bomb
aimer's station, a bomb bay and a dorsal gun turret. Little time was
lost by either the Bristol company or the Air Ministry, for following
the first flight of the prototype, on 25 June 1936, it was moved to
Boscombe Down on 27 October 1936 for the start of official trials, with
initial deliveries to RAF squadrons beginning in March 1937. In July
1937 the Air Ministry placed a follow-on order for 434 additional
Blenheim Mk Is, as the type had by then been named.
all-metal construction, except for fabric-covered control surfaces, the
Blenheim Mk I was a cantilever mid-wing monoplane, with the wing having
Frise mass-balanced ailerons and split trailing-edge flaps. The
fuselage nose extended only slightly forward of the engines, and both
fuselage and tail unit were conventional light alloy structures.
Landing gear was of the retractable tailwheel type. The tailwheel of
the prototype had retracted, operated by cables linked to the main
landing gear but, wisely, this feature was not carried forward into the
production aircraft. The powerplant comprised two Bristol Mercury VIII
engines developing 730 hp (545 kW) for take-off with a maximum power
rating of 840 hp (626 kW) in level flight, mounted in nacelles on the
wing leading-edge, and driving three-blade variable-pitch propellers.
Accommodation was provided for a pilot, navigator/bomb-aimer, and air
gunner/radio operator. A bomb bay in the wing centre-section could
contain a maximum 1,000 Ibs (454 kg) of bombs, and standard armament
comprised a 7.7 mm (0.303 in) machine-gun in the port wing, plus a
single Vickers 'K' machine-gun in a dorsal turret.
Initial deliveries of production Blenheim Mk Is to the RAF squadrons
began in March 1937. The first aircraft (K7036) to be delivered,
however, crashed upon landing totally destroying the aircraft. The
first RAF squadron to receive Blenheim Mk Is was No.114, then based at
RAF Wyton, and it was this unit which first demonstrated the new type
officially to the public at the RAF's final Hendon Display in the
summer of 1937. The Blenheims were to arouse excited comment with their
high speed and modern appearance, being launched on their career in an
aura of emotion created by the belief that, in an unsettled Europe, the
RAF was armed with the world's most formidable bomber aircraft.
Production contracts soared, necessitating the establishment of new
construction lines by A. V. Roe at Greengate, Middleton (Chadderton)
and Rootes Securities at Speke (South Liverpool), both these factories
being in Lancashire. Between them the three lines built a total of
1,355 Blenheim Mk Is which, at their peak, equipped no fewer than 26
RAF squadrons at home and overseas, the Blenheim's first overseas
deployments being with No.30 Squadron in Iraq and No.11 Squadron in
India, in January and July 1938 respectively.
However, by the outbreak of World War II few Blenheim Mk Is remained in
service with home-based bomber squadrons, having been superseded in the
bombing role by the Blenheim Mk IV, which incorporated the lessons
learned from the experience which squadrons had gained in operating the
Mk I. But their usefulness was by no means ended, many continuing to
serve as conversion trainers and, initially, as crew trainers in OTUs.
More valuable by far were some 200 which were converted to serve as
night fighters, pioneering the newly conceived technique of AI
(Airborne Interception) radar, carrying AI Mk III or Mk IV. The single
forward-firing machine-gun was totally inadequate for this role, of
course, and a special underfuselage pack to house four 7.7 mm (0.303
in) machine-guns was produced. So equipped, Blenheim Mk IFs scored the
first AI success against an enemy aircraft on the night of 2-3 July
Export versions of the Blenheim Mk I were sold before the war to
Finland, Turkey and Yugoslavia, and were also built under licence by
these first two nations. In addition, a small number had been supplied
to Romania as a diplomatic bribe in 1939, but this proved to be
unsuccessful. The result, of course, was that Blenheim Mk Is fought for
and against the Allies.
When, in August 1935, the Air Ministry had initiated Specification
G.24/35 to find a successor to the Avro Anson for use in a coastal
reconnaissance/light bomber role. Bristol had proposed its Type 149.
Very similar to the Blenheim Mk I, this was based on the use of Bristol
Aquila engines to confer long range with the existing fuel capacity,
but proved unacceptable to the Air Ministry. Subsequently renewed
interest was shown in the Type 149 for use in a general reconnaissance
role, and a prototype was built, by conversion of an early Blenheim Mk
I, this retaining the Mercury VIII engines and being provided with
increased fuel capacity. The fuselage nose was lengthened to provide
additional accommodation for the navigator/observer and his equipment,
and this was to be finalised as that which graced the Blenheim Mk IV.
Air Ministry then had misgivings about the Type 149, fearing that its
introduction and manufacture would interfere with the production or
urgently needed Blenheims. Instead, the Type 149 was adopted by the
Royal Canadian Air Force and with the start of the British Commonwealth
Air Training Plan, it was decided to produce a version in this country.
Fairchild Aircraft of Longueil outside Montreal was selected to produce
them under the Canadian name Bolingbroke. Quickly nicknamed the Boly,
the type saw service throughout Canada. The Bristol prototype being
shipped to Canada to help in the establishment of a production line.
The first Bolingbroke Is had Mercury VIII engines, but after 18 of
these had been built production changed to the definitive Canadian
version, the Bolingbroke IV with Mercury XV engines, and equipment from
both Canadian and US manufacturers. Later variants included a small
number of Bolingbroke IV-Ws with American built Pratt and Whitney Twin
Wasp Junior (SB4-G) 14-cylinder engines rated at 825 hp (615 kW) for
take-off, and a number of Bolingbroke IV-T multi-purpose trainers.
Having blown hot and then cold over the Type 149, there was a sudden
renewal of interest, primarily as an interim measure until the Type 152
torpedo-bomber, derived from the Blenheim, should become available. The
decision was taken, therefore, to introduce the longer nose and stepped
windscreen of the Bolingbroke, and to make provision for longer range
by the introduction of increased wing fuel capacity. The Bristol
designation Type 149 was retained for this changed configuration, the
new RAF designation being Blenheim Mk IV. This change took place
quietly on the production lines towards the end of 1938, although the
first 68 Blenheim Mk IVs were built without the 'Iong-range wing'. The
powerplant comprised two more powerful Mercury XV engines, and these
allowed gross weight to be increased eventually by 16 per cent.
90 Squadron was the initial unit to be equipped with Blenheim Mk IV s
in March 1938, the first of more than 70 squadrons to operate these
aircraft, and consisting of units from Army Co-operation, Bomber,
Coastal, Far East Bomber, Fighter and Middle East Commands, both at
home and overseas. Inevitably, such extensive use brought changes in
armament and equipment, but especially the former, for the armament of
the first Blenheim Mk IVs was unchanged from the initial two-gun
armament of the Mk I. As finalised the number became five, the single
forward-firing gun in the wing being retained, a new dorsal turret
carrying two guns being adopted, and a completely new
remotely-controlled Frazer-Nash mounting being added beneath the nose
to hold two aft-firing machine-guns. Protective armour was also
increased, but while it was not possible to enlarge the capacity of the
bomb bay, provision was made for an additional 320 Ibs (145 kg) of
bombs to be carried externally, under the inner wings, for short-range
With so many squadrons operating the type it was inevitable that
Blenheims should notch up many wartime 'firsts' for the RAF. These
included the first reconnaissance mission over German territory, made
on 3 September 1939. It was flown by Flying Officer A Macpherson in a
Blenheim (N6215) Mk IV of No. 139 Squadron while on an armed
reconnaissance over German warships at Heligoland Bight (Schillig
Roads) near Wilhelmshaven. On 4 September 1939, ten aircraft from Nos.
107 & 110 Squadrons, led by Flight Lieutenant K.C. Doran of No. 110
Squadron made an attack on the same German ships. From the beginning of
the war, until replaced in home squadrons of Bomber Command by Douglas
Bostons and de Havilland Mosquitoes in 1942, Blenheim Mk IVs were used
extensively in the European theatre. Although vulnerable to fighter
attack, they were frequently used for unescorted daylight operations
and undoubtedly the skill of their crews and the aircraft's ability to
absorb a great deal of punishment were the primary reasons for their
survival, for high speed and heavy firepower was certainly not their
forte. In the overseas squadrons Blenheims continued to serve long
after their usefulness had ended in Europe, and except in Singapore,
where they were no match for the Japanese fighters, they proved a
valuable weapon. A total of 3,298 Mk IV had been built in England when
production ended, and in addition to serving with the RAF had been used
by the French Free and South African air forces, and supplied in small
numbers to Finland, Greece and Turkey.
Last of the direct developments of the Blenheim design was Bristo1's
Type 160, known briefly as the Bisley, which was to enter service in
the summer of 1942 as the Blenheim Mk V. Envisaged originally as a
low-altitude close-support bomber, it was in fact to be built for
deployment as a high-altitude bomber, powered by Mercury XV or XXV
engines. Except for a changed nose, some alterations in detail and
updated equipment, these aircraft were basically the same as their
predecessors. Some 942 were built, all produced by Rootes at their
Speke (South Liverpool) and Blythe Bridge (Stoke-on-Trent) factories,
and the first unit to receive Blenheim Mk Vs was No.18 Squadron. The
type was to equip six squadrons in the Middle East and four in the Far
East, where they were used without distinction. This resulted from an
increase in gross weight of over 17 per cent which, without the
introduction of more powerful engines, had brought about a serious fall
of performance. It only when the Blenheim Mk Vs were deployed in
Italian campaign, contending with the advanced fighters in service with
the Luftwaffe, that losses rose to quite unacceptable proportions, and
the Blenheim Mk Vs withdrawn from service.
Operational use of the Bolingbroke was limited to the Royal Canadian
Air Force in Canada and the Aleutian Islands. No 8 (Bomber
Reconnaissance) Squadron was the first RCAF unit to convert to the
Bolingbroke, followed by one other squadron. Bolingbrokes were used
primarily to fly anti-submarine coastal patrols over both the Atlantic
RCAF squadrons were assigned to the combined American-Canadian defence
campaign to protect the Aleutian Islands and west coast of Alaska from
Japanese attack. No. 115 Squadron arrived in the Aletuians in April of
1942 and was assigned anti-submarine patrol and maritime reconnaissance
missions. In June of 1942 No. 8 Squadron deployed to the Aleutians with
twelve Bolingbroke Mk IVs, making a 1,000 mile flight from RCAF Sea
Island to Yakutat Island arriving on 3 June. When the squadron arrived
it was ordered to paint out the Red centers to the upper wing roundels
to prevent confusion with the Japanese 'meatball' insignia. Later
additional recognition markings in the form of a fourteen inch Blue
band was added to the rear of the fuselage. The harsh weather in the
Aleutians proved a worse enemy than the Japanese and a number of
Bolingbrokes were lost when thick Alaskan fogs obscured mountain tops.
Normal bomb loads consisted of three 300 pound depth charges and two
aircraft were maintained in an alert status at all times. The squadron
is credited with sharing one submarine kill with the US Navy. A
Bolingbroke Mk IV piloted by Flight Sergeant P.M.G. Thomas attacked and
damaged a Japanese submarine enabling US Navy surface units to later
majority of Bolingbrokes produced never saw combat, instead they
performed as crew and operational trainers under the Commonwealth Air
Training Plan, training crews for overseas units. Still others were
converted to unarmed target tugs with high visibility paint schemes for
training air gunners and army anti-aircraft gunners.
Specifications (Bristol Type 149 Blenheim Mk IV)
Type: Three Seat Light Bomber, Fighter & Night Fighter, Maritime
Reconnaissance (Anti-Shipping/Submarine), Bombing and Gunnery Trainers
& Target Tug.
Accommodation/Crew: Pilot, Navigator/Bomb-Aimer and Wireless
Operator/Air Gunner. See the Blenheim Mk I for more cockpit
Design: Chief Designer Frank Barnwell of the Bristol Aeroplane
Manufacturer: The Bristol Aeroplane Company Limited based at Filton
(Bristol), Bristol County, England (Mark I, IV & V Prototypes),
Alexander V. Roe (Avro) Aircraft Company Limited based in Greengate,
Middleton (Chadderton), Lancashire County, England (Mark I & IV),
Rootes Securities Limited at Blythe Bridge (Stoke on Trent),
Straffordshire County, England (Mark IV & V), Rootes Securities Limited
at Speke (South Liverpool), Lancashire County, England (Mark I & V),
Fairchild Aircraft Limited in Longueil, Quebec, Canada (Bolingbroke).
Also built under licence by Valtion Lentokonetehdas (State Aircraft
Factory) at Tampere, Finland (Mark I & IV) and Ikarus AD in Belgrade (Zemun),
Yugoslavia (Mark I).
Powerplant: (100 Octane Fuel) Two Bristol Mercury XV 9-cylinder
poppet-valve air-cooled radial engines developing 905 hp (675 kW) at
take-off, a maximum output of 995 hp (742 kW) for level flight (5
minute usage) and a maximum ecomical cruising power output of 590 hp
(440 kW) at 16,000 ft (4877 m) at 2400 rpm. (87 Octane Fuel) Two
Bristol Mercury XV 9-cylinder poppet-valve air-cooled radial engines
developing 725 hp (541 kW) at take-off, a maximum output of 840 hp (627
kW) for level flight (5 minute usage) and a maximum ecomical cruising
power output of 590 hp (440 kW) at 16,000 ft (4877 m) at 2400 rpm.
Performance: Maximum speed 266 mph (428 km/h) at 11,800 ft (3595
m); cruising speed of 198 mph (319 km/h); service ceiling (clean)
27,260 ft (8310 m) or 22,000 ft (6706 m) fully loaded; initial climb
rate 1,480 ft/min (7.5 m/sec).
Fuel Capacity: Two inboard 140 Imperial gallon (636 litres) main
fuel tanks and two outboard 94 Imperial gallon (427 litres) auxiliary
or long range fuel tanks giving a total capacity of 468 Imperial
gallons (2125 litres). Starting in early 1940 the main fuel tanks were
self-sealing, but due to an initial shortage, the outboard auxiliary
fuel tanks remained non self-sealing for some time.
Oil Capacity: One 11.5 Imperial gallon (52.2 litre) main oil tank
and a 2.5 Imperial gallon (11.3 litre) auxiliary oil tank per engine
giving a total oil capacity of 28 Imperial gallons (127.2 litres).
Range: 1,460 miles (2350 km) on internal fuel with a 1,000 lbs (454
kg) bombload. 1,950 miles (3140 km) on internal fuel without bombs.
Weights & Loadings: Empty 9,790 lbs (4441 kg), with a normal
take-off weight of 13,500 lbs (6122 kg) and a maximum take-off weight
of 14,400 lbs (6532 kg) fully loaded with bombs.
Dimensions: Span 56 ft 4 in (17.17 m); length 42 ft 7 in (12.98 m);
height 9 ft 10 in (3.00 m); wing area 469 sq ft (43.57 sq m).
Defensive Armament: A total of three to five 7.7 mm (0.303 in)
machine-guns was standard. Some Mk IV aircraft underwent various field
modifications with further increased the aircrafts defensive armament.
The Browning machine-guns were belt feed while the Vickers machine-guns
used 50 round circular ammunition pans. The Frazer-Nash FN.54 and
FN.54A turrets were jettisonable in the event of an emergency allowing
the crew to use the lower fuselage emergency escape hatch.
× 7.7 mm (0.303 in) Browning fixed forward-firing machine-gun in the
x 7.7 mm (0.303 in) Lewis or Vickers "K" trainable machine-gun in a
semi-retractable hydraulically operated Bristol B.Mk III dorsal
x 7.7 mm (0.303 in) Vickers "K" trainable machine-guns in a
power-operated Bristol B.Mk IIIA dorsal turret, or
x 7.7 mm (0.303 in) Browning trainable machine-guns in a
power-operated Bristol B.Mk IV dorsal turret.
× 7.7 mm (0.303 in) Browning trainable rearward-firing machine-gun in
a remotely controlled Frazer-Nash FN.54 chin turret, or
× 7.7 mm (0.303 in) Browning trainable rearward-firing machine-guns in
a remotely controlled Frazer-Nash FN.54A chin turret. The turret could
rotate 20 degrees to either side with a depression of 17 degrees.
× 7.7 mm (0.303 in) Vickers "K" forward-firing machine-gun in a gimbal
nose gun mount (optional field modification).
× 7.7 mm (0.303 in) Vickers "K" machine-gun in a rear firing engine
nacelle mount (optional field modification).
× 7.7 mm (0.303 in) Vickers "K" machine-gun in a rear firing under
tail mount (optional field modification).
Offensive Armament: 1,000 lbs (454 kg) of bombs internally and up
to 320 lbs (145 kg) of bombs externally on two underwing racks located
between the fuselage and engine nacelles. On the single Mk II aircraft
produced, 500 lbs (227 kg) of bombs could be carried externally but at
great expense to performance.
× 250 lbs (114 kg) bombs, or
× 500 lbs (227 kg) bombs, or
× 300 lbs (114 kg) depth charges carried internally.
× 80 lbs (36.2 kg) bombs on underwing racks, or
× 160 lbs (72.5 kg) bombs on underwing racks
Variants: Bristol Type 142, Bristol Type 142M, Bristol Type 143
(Aquila engined), (Type 142M) Blenheim Mk I Prototypes, (Type 142M)
Blenheim Mk I, Blenheim Mk IF, Blenheim PR.Mk I, Blenheim Mk II,
Blenheim Mk III, Bristol Type 149, (Type 149) Blenheim Mk IV
Prototypes, (Type 149) Blenheim Mk IV, (Type 149) Blenheim Mk IVF,
(Type 149) Blenheim Mk IVL, (Type 160) Bisley Mk I, (Type 160) Blenheim
Mk V, Bolingbroke Mk I, Bolingbroke Mk II, Bolingbroke Mk III,
Bolingbroke Mk IV, Bolingbroke Mk IV-C, Bolingbroke Mk IV-W,
Bolingbroke Mk IV-T, Bolingbroke Mk IV-TT
Equipment/Avionics: Standard communications and navigation
History: First flight (Type 142 "Britain First") 12 April 1935;
first flight (Type 142M) 25 June 1936; initial delivery (No. 114
Squadron RAF) March 1937; end production (VD) June 1943; withdrawn from
service (Finland) 1956.
Operators: United Kingdom (RAF), Canada (RCAF), Finland, Turkey,
Yugoslavia, Romania, Greece (Royal Hellenic Air Force), Free French Air
Force (Forces Francaises Libre), Portugal (Arma de Aeronautica), South
Africa (SAAF) and Croatia. The German Luftwaffe and the Italian Regia
Aeronautica both operated captured aircraft.
Number Built: 4,422
Number Still Airworthy: One