Formal 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).
Only one Blenheim Mk II was completed, as flight tests revealed the increase in speed to be marginal and not warranting further development.
Another modification resulted in the Blenheim Mk III, which lengthened the nose, dispensing with the "stepless cockpit" format of the Mk.
I, introducing a true windscreen in front of the pilot, to provide more room for the bomb aimer.
This required the nose to be "scooped out" in front of the pilot to maintain visibility during takeoff and landing.
Both modifications were combined, along with a newer version of the Mercury engine with 905 hp (675 kW).
The turret acquired a pair of Brownings in place of the original single Vickers K gun, creating the Blenheim Mk IV.

In early 1939, the first batch of Blenheim Mk IVs were accepted into service; these lacked outer fuel tanks but were accepted due to the urgent demand for the type.
Early Blenheim Mk IVs were also equipped with the Mercury VIII engine, most were fitted with the more powerful Mercury XV or Mercury 25 models.
Further aircraft deliveries were made to the production standard and were primarily manufactured by Avro and Roots.
Production of the Blenheim IV continued until June 1943, when newcomers such as the Beaufort-derived Beaufighter had succeeded the type.
A total of 3,307 were produced.

A long-range fighter version, the Blenheim Mk IF, was also developed.
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 Blenheim had been selected as the first aircraft to be adapted for this role as its fuselage was sufficiently roomy to accommodate the additional crew member and radar apparatus.
Their performance was marginal as a fighter but they served as an interim type pending availability of the more capable Beaufighter derivative.
About 60 Mk IVs were also equipped with the gun pack as the Mk IVF and were used by Coastal Command to protect convoys from German long-range bombers.

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 bomb-aimer position and another Mercury variant 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 (Type 160) was used primarily in the Middle East and Far East.
The Blenheim served as the basis for the Beaufort torpedo bomber, which led to the Beaufighter, with the lineage performing two evolutions of bomber-to-fighter.
Formal 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). Only one Blenheim Mk II was completed, as flight tests revealed the increase in speed to be marginal and not warranting further development. Another modification resulted in the Blenheim Mk III, which lengthened the nose, dispensing with the "stepless cockpit" format of the Mk.I, introducing a true windscreen in front of the pilot, to provide more room for the bomb aimer. This required the nose to be "scooped out" in front of the pilot to maintain visibility during takeoff and landing. Both modifications were combined, along with a newer version of the Mercury engine with 905 hp (675 kW). The turret acquired a pair of Brownings in place of the original single Vickers K gun, creating the Blenheim Mk IV.
In early 1939, the first batch of Blenheim Mk IVs were accepted into service; these lacked outer fuel tanks but were accepted due to the urgent demand for the type. Early Blenheim Mk IVs were also equipped with the Mercury VIII engine, most were fitted with the more powerful Mercury XV or Mercury 25 models. Further aircraft deliveries were made to the production standard and were primarily manufactured by Avro and Roots. Production of the Blenheim IV continued until June 1943, when newcomers such as the Beaufort-derived Beaufighter had succeeded the type. A total of 3,307 were produced.
A long-range fighter version, the Blenheim Mk IF, was also developed. 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 Blenheim had been selected as the first aircraft to be adapted for this role as its fuselage was sufficiently roomy to accommodate the additional crew member and radar apparatus. Their performance was marginal as a fighter but they served as an interim type pending availability of the more capable Beaufighter derivative. About 60 Mk IVs were also equipped with the gun pack as the Mk IVF and were used by Coastal Command to protect convoys from German long-range bombers.
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 bomb-aimer position and another Mercury variant 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 (Type 160) was used primarily in the Middle East and Far East. The Blenheim served as the basis for the Beaufort torpedo bomber, which led to the Beaufighter, with the lineage performing two evolutions of bomber-to-fighter.
Bristol Blenheim