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Blenheims marked the first RAF violation of German airspace in the war when a flight crossed over into enemy territory on a reconnaissance mission near Wilhelmshaven (September 3rd, 1939). The next day, Blenheims attacked German naval positions at the Elbe Estuary but this force of ten was reduced to just five returning bombers and many of the dropped ordnance failed to detonate. The aircraft was continually pushed into action despite its shortcomings and spent time in the skies above Norway, France and the Low Countries. In these environments the aircraft proved its underperforming self and lacked suitable defensive capabilities - under-powered and under-gunned she was. This led to revisions of the armament suite, namely increasing the overall machine gun count carried. On March 11, 1940, a Blenheim claimed the RAF's first sinking of an enemy submarine (a German U-Boat).

The Blenheim equipped the first night fighter unit anywhere in the world when Squadron No.25 was given the type. During the "Battle of Britain" in the summer of 1940, a Blenheim night fighter claimed the first aerial kill while carrying radar when it downed a raiding German Dornier Do 17 Medium Bomber. The series played a crucial role in the night defense of Britain proper despite its inherent deficiencies in both performance and firepower.

The series soldiered on in anti-shipping sorties across the North Sea and in daring low-altitude runs against German infrastructure when possible. Overseas units were featured in the Middle East, Far East, North African and Mediterranean theaters (including time over Greece and Crete). Finland, becoming the first export customer of the Blenheim back in 1936 (both through direct order and local, licensed production), showcased the light bomber during its wars against the Soviet Union (the "Winter War" and the "Continuation War").

Origins of the Bristol Blenheim Light Bomber / Heavy Fighter place it in the 1930s when Lord Rothermere, owner of the "Daily Mail" newspaper, set out a challenge to produce the fastest aircraft in Europe. From this came the Bristol "Type 142" with its sleek design holding much promise. It certainly went on to impress onlookers including Royal Air Force authorities who eventually considered it for military service (indeed it proved faster that some frontline fighters of the day). Specification B.28/35 was drawn up by the Air Ministry in 1935 to cover the conversion process of the fast plane to a three-seat, twin-engined light bomber / heavy fighter form. It was known to Bristol as the "Type 142M" and named Blenheim in RAF service. An initial order of 150 in 1935 followed and this was strengthened by a 1936 order for 434 more (as the Blenheim Mk.I).

War time Revisions
The next major modification resulted in the Blenheim Mk III, which lengthened the nose, and thereby dispensed with the "stepless cockpit" format of the Mk.I in introducing a true windscreen in front of the pilot, to provide more room for the bombardier. This required the nose to be "scooped out" in front of the pilot to maintain visibility during takeoff and landing. Work on an extended-range reconnaissance version started as the Blenheim Mk II, which increased tankage from 278 gal (1,264 L) to 468 gal (2,127 L), but only one was completed. However both of these modifications were instead combined, along with a newer version of the Mercury engine with 905 hp (675 kW) and the turret acquired a pair of Brownings instead of the Vickers K; creating the Blenheim Mk IV. Another modification led to a long-range fighter version; the Blenheim Mk IF. For this role, about 200 Blenheims were fitted with a gun-pack under the fuselage for four .303 in (7.7 mm) Brownings. Later, the Airborne Intercept (AI) Mk III or IV radar was fitted to some aircraft in use as night fighters; these were the first British fighters to be equipped with radar. The last bomber variant was conceived as an armoured ground attack aircraft, with a solid nose containing four more Browning machine guns. Originally known as the 'Bisley', (after the shooting competitions held at Bisley), the production aircraft were renamed Blenheim Mk V and featured a strengthened structure, pilot armour, interchangeable nose gun pack or bombardier position, and yet another Mercury variant, this time with 950 hp (710 kW). The Mk V was ordered for conventional bombing operations, with the removal of armour and most of the glazed nose section. The Mk V, or Type 160, was used primarily in the Middle East and Far East.

Initial users were the airmen of Squadron No.114 during March of 1937 and a stock of hundreds were available when Britain went to war in September of 1939. However, of this total many were already stationed overseas in far-off places like the Middle East and Far East so they held little value in defending the home front and in attacking Axis positions across mainland Europe. The line was continually evolved throughout its early involvement in the war and this resulted in a few more specialized designs including a night fighters and several long-range performers.

The line eventually served out its usefulness and was gradually replaced from 1942 onward by such designs as the American Douglas "Boston" and British de Havilland Mosquitoes and Bristol Beaufighters. The RAF gave up use of all Blenheims as soon as 1944.

Like many aircraft designed and built prior to the war, bombers were generally faster than the biplane fighters then in service and were considered wonder weapons. The Germans fell into this category, believing that their fast flying Junkers Ju88 and Heinkel He111 could outspeed all opposition, negating the requirements for fighter escort. Each nation had to learn that bombers without escorts flying into enemy airspace controlled by a modern defence fighter force would suffer grevious losses. The Germans in the Battle of Britain, the daylight raids by Bomber Command, the daylight raids by the 8th Air Force, until escorting fighters could travel with them to Berlin and back, clearing all before them. The Blenheim was a tactical bomber which needed escorts, however, the Briitsh sent them on many a raid without them and as such, losses of the Blenheim were very high on these raids.

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.