Bristol Blenheim, part eight. The Blenheim goes to war

 

An extended range reconnaissance version of the Blenheim was worked on, named as "Blenheim Mk.II". This was a regular Mk.I with the turret removed and larger fuel tanks. Only one prototype was built as, when tested, performance rates were  only marginally better and the variant was discarded.
Another Mk.III version was also developed which had a longer nose with the steepless cockpit of the Mk.I with a true windscreen in front of the pilot, giving this way, more room for the bomb aimer. This configuration required the nose to be "scooped out" in front of the pilot to keep visibility during take off and landing. This machine was further developed and, together with a newer version of the 905 hp Mercury radial engine and a pair of Browning machine guns (instead of the original Vickers K gun) placed in the turret, led to the Mk.IV variant. 
During the first half of 1939 the first production batch of Mk.IVs were accepted into service with the Royal Air Force (RAF). They lacked, however, outer fuel tanks, due to the urgent demand of the type and were powered by an earlier variant of the Mercury engine. From 1939 until June 1943 (though some sources claim it was until 1944) a total of 3.307 Mk.IVs were manufactured including those manufactured in Finland.

A long-range fighter variant was also developed, the Mk.IF. These machines were fitted with a gun pack containing four .303 in Browning machine guns. 

In September 1939, when the World War 2 broke out, the Blenheim Mk.I was serving with two home-based squadrons and 11 overseas ones in places such as Egypt, Iraq, Aden, India and Singapore. Further bomber squadrons were in the process of converting to the more capable Mk.IV and, by the outbreak of the war, a total of 168 Blenheim Mk.IV were in active service with the RAF. 

On 3rd September 1939, the day when both Britain and France declared war on Germany, a single Mk.IV piloted by Flying Officer Andrew McPherson was the first British airplane to fly over Germany to perform a reconnaissance mission on the Kriegsmarine's (German Navy) facilities in Wilhelmshaven, Lower Saxony. The next morning, fifteen Blenheims belonging to three different squadrons, took off to bomb the ships spotted the previous morning. RAF's Coastal Command also employed the type early on with the stated mission of protecting shipping convoys off the east coast of Britain. 

Sources:

1. https://en.wikipedia.org/wiki/Bristol_Blenheim

2. Hall Park Books - Warpaint 26 - Bristol Blenheim

3.Osprey Publishing - Combat Aircraft 5 - Blenheim Squadrons of World War 2