When the twin-engined Blenheim bomber first appeared its modern appearance and speed installed the belief that the Royal Air Force possessed the best bomber in the world. But when World War 2 broke out in September 1939 the type suffered heavy losses during unescorted daylight raids during the early part of the conflict. Despite this the Bristol Blenheim served the RAF for seven years at home and overseas.

The owner of the Daily Mail during the 1930s Lord Rothermere had a very keen interest in aviation spanning two decades and in 1934 wanted an aircraft for his own use. The aircraft would need to have space for 6 passengers and 2 crew members with a good top speed. By coincidence a design outline for a two-engined light transport aircraft to be powered by the, currently in development, 500-hp Bristol Aquila 1 had been done by Frank Barnwell at the Bristol Aeroplane Company. However with Lord Rothermere's requirement for speed the Aquila 1s would be replaced by the 650-hp Bristol Mercury VIS powerplant.

This new aircraft made its maiden flight on the 12th April 1935 from Filton and the Type 142, as it was known, was to be faster than any Royal Air Force fighter currently in service. Unsurprisingly the Air Ministry were interested in the aircraft and after being allowed to evaluate the type it was revealed to the nation and was called 'Spirit of Britain'

With the Air Ministry looking at the aircraft as a potential light bomber Bristol would work on a militarised version designated the Type 142M. The main changes made to the design was the addition of a dorsal turret, bomb bay and bomb aimer's station. The company submitted this idea to the Air Ministry which they accepted. So under Specification 28/35 an initial order for 150 was placed in September 1935.

Named the Blenheim and of all metal construction bar the control surfaces, which were fabric covered, the prototype made its first flight on the 28th June 1936. The Mk I had another change of engines with power supplied by a pair of 840-hp Bristol Mercury VIII engines with three blade variable pitch propellers giving the aircraft a top speed of 285 mph. Featuring retractable landing gear and tail wheels, although the production versions would be without the retractable tail wheel. Armament consisted of a sole 0.303-in machine-gun located in the port wing whilst the dorsal turret had a Vickers 'K' gun and a bomb load of 1,000lb. The Blenheim would have a crew of 3 consisting of pilot, navigator/bomb-aimer and air gunner/radio operator.

Deliveries of the first Blenheims started around nine months after the prototype flight when No. 114 Squadron, Wyton received theirs on the 1st March 1937. This was followed four months later in July by an order for a further 434 aircraft. It was whilst with No. 114 Squadron that the public got their first glimpse of the type when it displayed at the Royal Air Force Hendon display on the 27th June 1937. With its sleek appearance and speed the Blenheim excited people and installed the belief that the RAF possessed the best bomber in the world.

With an influx of new orders for the Blenheim two new additional construction lines, both in Lancashire, were set up. One at A.V Roe, Chadderton and the other at Rootes Securities, Speke. In total 1,552 Mk Is would be built and would serve with around 26 Squadrons both at home and aboard. However by the time the Second World War broke out in September 1939 only a handful of Mk Is were being used as bombers by home based squadrons, the type being moved into other roles. Some would serve as both crew and conversion trainers whilst 200 were converted to night fighters and designated Blenheim Mk IF. This would see the aircraft fitted with additional armament in the form of four 0.303-in machine-guns fitted in a pack underneath the fuselage. Also installed was either Mk III or Mk IV Airborne Interception radar and it was a Mk IF of the Fighter Interception Unit at RAF Tangmere, West Sussex which, on the 22nd July 1940, shot down the first enemy aircraft using AI, a Dornier Do 17.

In August 1935, just four months after the Blenheim first flew, the Air Ministry issued Specification G.24/35 which required an aircraft which would be used for coastal reconnaissance and as a light bomber and was intended to replace the Avro Anson. The design Bristol submitted was based on the Blenheim Mk I and was known as the Type 149. With the installation of Bristol Aquila engines it was intended to increase the range of the aircraft without the need for additional fuel tanks. However this design was rejected by the Air Ministry.

Despite this initial rejection the Air Ministry again looked at the Type 149 design but as a general-reconnaissance aircraft. This would see a Mk I used as the base with a number of changes made. Firstly the nose of the fuselage was increased so the navigator/observer had more room, additional fuel tanks added and the Mercury VIII engines were kept instead of the intended Aquila powerplant. This flew for the first time on the 24th September 1937. With the pressing need for Blenheims there was concern at the Air Ministry that by putting the Type 149 into production it would hinder building of the Blenheim. Instead the aircraft would be sent to Fairchild Aircraft at Longueuil, Quebec in Canada where the type would be built for the Royal Canadian Air Force and named the Bolingbroke.

Another change of heart by the Air Ministry saw them again consider the Type 149 this time as a stop gap as a torpedo-bomber until, another Bristol design, the Beaufort was able to enter service. These aircraft would be known as the Blenheim Mk IV and featured bigger fuel tanks in the wings and would have the Bolingbroke's longer nose design. Power would be supplied by a pair of 920-hp Mercury XV engines giving a top speed of 266 mph. These modifications to the aircraft would begin in late 1938 although the first 68 wouldn't have the bigger fuel tanks.

The new Mk IV would enter service during March 1938 with armament consisting of two machine-guns. Over time this would increase to five with the addition of a dorsal turret and underneath the nose a Frazer-Nash mounting which was controlled remotely. The bomb load was increased by bomb racks under the wings which could hold 320lb bombs in addition to the 1,000lb in the bomb bay. The Blenheim Mk IV would see widespread service not only with Bomber Command but also Fighter and Coastal Command among others all over the world serving with over 70 squadrons.

Blenheims would be in action on the same day Britain declared war on Germany, 3rd September 1939, when a sole Mk IV from No. 139 Squadron flew over Germany on a reconnaissance mission over Wilhelmshaven to monitor the German Fleet and the type also became the first British aircraft to attack German targets when Blenheims raided the German fleet off Wilhelmshaven. Two squadrons Nos. 114 and 139 would be sent to France as part of the Royal Air Force's Advanced Air Striking Force and would operate alongside the Fairey Battle. Losses for the aircraft would be high as they operated during daylight and not always with fighter escort. One such mission on the 14th May 1940 saw five out of eight Blenheims fail to return from a raid on the Sedan bridgehead.

After 'Operation Dynamo', the evacuation of Dunkirk, the Battle of Britain would begin and the Blenheim and its crews would undertake raids on German airfields in occupied Europe and invasion barges located in occupied ports. As with some of their missions in France the aircraft would be operating unescorted in daylight hours. Losses again were high with one raid on Aalborg airfield, Denmark on the 13th August 1940 seeing 100% losses when none of the 11 Blenheims sent returned.

With the introduction of newer and faster aircraft the Blenheims time with home based Bomber Command squadrons came to an end in 1942, this wasn't the end of the types front line service as they continued to serve overseas, not only with the Royal Air Force but other air forces including the Free French and Finnish.

The final Blenheim to be produced was the MK V, which for a while was named the Bisley and also known as the Type 160, it was intended for this aircraft to be used as a close support bomber. With only a few modifications and powered by either Mercury XV or XXV engines the Mk V was essentially the same as previous Mks. However these changes saw the aircraft when produced serve as a high-altitude bomber. 10 overseas squadrons based in the Middle East and Far East would receive the Mk V with deliveries beginning during the summer of 1942 but when moved to serve in the Italian campaign the Luftwaffe with its modern fighters inflicted heavy losses on the Blenheim. In part this was due to the fact that the extra weight had a detrimental effect on performance as the MK V had a top speed of 262 mph some 34 mph down on its predecessor the Mk IV. So by 1944 the type was no longer in RAF service and a total of 4,422 had been built.