Formal work on an extended-range reconnaissance version started as the Blenheim Mk II, which increased tankage from . 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 Both modifications were combined, along with a newer version of the Mercury engine with . The turret acquired a pair of Brownings in place of the original single Vickers K gun, creating the Blenheim Mk IV.