In September 1939, the month in which the conflict that would become known as the Second World War broke out, the Blenheim Mk I equipped a total of 2 home-based squadrons as well as 11 overseas squadrons in locations such as Egypt, Aden, Iraq, India, and Singapore.
Further RAF squadrons had also received, or were in the process of converting to, the more capable Blenheim Mk IV; a total of 168 Blenheim Mk IV aircraft had entered RAF operational strength by the outbreak of war.

On the day that war was declared on Germany, a Blenheim Mk IV, N6215, piloted by Flying Officer Andrew McPherson was the first British aircraft to cross the German coast to perform a high altitude reconnaissance mission upon the German Navy in the vicinity of Wilhelmshaven, Lower Saxony.
The following morning, 15 Blenheims from three squadrons set off on one of the first bombing missions to attack the ships spotted on the previous day.
RAF Coastal Command were soon using the Blenheim with the stated mission of protecting British shipping convoys off the east coast.

Shortly after the conflict's start, the RAF Advanced Air Striking Force (AASF) was deployed to numerous airfields in France, allowing for shorter range bombing missions against German targets, including industries.
Several squadrons of Blenheim IVs were assigned to the AASF, being frequently used against targets in France and the Low Countries once the Battle of France had begun.
Blenheims were also assigned to the air component of the British Expeditionary Force of the Army.

In May 1940, AASF and BEF Blenheims participated in the Battle of France, being sent against German forces moving towards Brussels, resulting in many aircraft quickly sustaining heavy damage or being lost to enemy fire.
German attacks upon the French airfields also damaged a considerable number of Blenheims on the ground.
On 14 May, a combined force of Fairey Battles and Blenheims was dispatched on a counter-attack upon German forces as they broke through defensive lines: 40 out of 71 aircraft were lost in this sortie.
This is claimed to be the highest ever losses known to the RAF.
Further action by Blenheims of Bomber Command that day sustained a 25% aircraft loss despite a high level of British fighter cover.
Shortly thereafter, the mostly-depleted squadrons were withdrawn to Britain.
Around 50 Blenheims supported the Dunkirk evacuation by harassing enemy forces.

Rapid advances in technology which had taken place in the late 1930s had rendered the Blenheim mostly obsolete by the outbreak of the war.
In particular, it had become heavier as extra service equipment was installed; much of this was found to be necessary through operational experience.
This, coupled with the rapid performance increases of the fighters that would oppose it, had eclipsed the Blenheim's speed advantage.
In January 1941, the Air Staff classified the Blenheim as inadequate in terms of performance and armament for current operations.

The light armament of one .303 in (7.7 mm) Vickers VGO in the turret and one .303 in (7.7 mm) Browning machine gun in the port wing was seldom able to deter fighter opposition.
Squadrons were forced to use several different improvisations in an attempt to provide better defensive armament, until officially sanctioned modifications were able to be introduced in early 1940.
The Blenheim also proved to be vulnerable to flak, especially around the rear fuselage.
Flexible, self-sealing liners had been fitted to the fuel tanks but they were still not fully protected against the 20 mm MG FF cannon carried by the Luftwaffe's Bf 109s and Bf 110s.
In September 1939, the month in which the conflict that would become known as the Second World War broke out, the Blenheim Mk I equipped a total of 2 home-based squadrons as well as 11 overseas squadrons in locations such as Egypt, Aden, Iraq, India, and Singapore. Further RAF squadrons had also received, or were in the process of converting to, the more capable Blenheim Mk IV; a total of 168 Blenheim Mk IV aircraft had entered RAF operational strength by the outbreak of war.
On the day that war was declared on Germany, a Blenheim Mk IV, N6215, piloted by Flying Officer Andrew McPherson was the first British aircraft to cross the German coast to perform a high altitude reconnaissance mission upon the German Navy in the vicinity of Wilhelmshaven, Lower Saxony. The following morning, 15 Blenheims from three squadrons set off on one of the first bombing missions to attack the ships spotted on the previous day. RAF Coastal Command were soon using the Blenheim with the stated mission of protecting British shipping convoys off the east coast.
Shortly after the conflict's start, the RAF Advanced Air Striking Force (AASF) was deployed to numerous airfields in France, allowing for shorter range bombing missions against German targets, including industries. Several squadrons of Blenheim IVs were assigned to the AASF, being frequently used against targets in France and the Low Countries once the Battle of France had begun. Blenheims were also assigned to the air component of the British Expeditionary Force of the Army.
In May 1940, AASF and BEF Blenheims participated in the Battle of France, being sent against German forces moving towards Brussels, resulting in many aircraft quickly sustaining heavy damage or being lost to enemy fire. German attacks upon the French airfields also damaged a considerable number of Blenheims on the ground. On 14 May, a combined force of Fairey Battles and Blenheims was dispatched on a counter-attack upon German forces as they broke through defensive lines: 40 out of 71 aircraft were lost in this sortie. This is claimed to be the highest ever losses known to the RAF. Further action by Blenheims of Bomber Command that day sustained a 25% aircraft loss despite a high level of British fighter cover. Shortly thereafter, the mostly-depleted squadrons were withdrawn to Britain. Around 50 Blenheims supported the Dunkirk evacuation by harassing enemy forces.
Rapid advances in technology which had taken place in the late 1930s had rendered the Blenheim mostly obsolete by the outbreak of the war. In particular, it had become heavier as extra service equipment was installed; much of this was found to be necessary through operational experience. This, coupled with the rapid performance increases of the fighters that would oppose it, had eclipsed the Blenheim's speed advantage. In January 1941, the Air Staff classified the Blenheim as inadequate in terms of performance and armament for current operations.
The light armament of one .303 in (7.7 mm) Vickers VGO in the turret and one .303 in (7.7 mm) Browning machine gun in the port wing was seldom able to deter fighter opposition. Squadrons were forced to use several different improvisations in an attempt to provide better defensive armament, until officially sanctioned modifications were able to be introduced in early 1940. The Blenheim also proved to be vulnerable to flak, especially around the rear fuselage. Flexible, self-sealing liners had been fitted to the fuel tanks but they were still not fully protected against the 20 mm MG FF cannon carried by the Luftwaffe's Bf 109s and Bf 110s.
Bristol Blenheim