Avro Lancaster B.I (Special)

Dropping Barnes Wallis's earthquake bombs meant stripping the bomber. The B.I (Special) received uprated engines with paddle-bladed propellers and shed weight and drag by losing turrets: first the dorsal, then two of the tail guns and the pilot's seat-back armour, and finally the nose turret. For the 12,000 lb (5,400 kg) Tallboy the bomb bay doors were bulged; for the 22,000 lb (10,000 kg) Grand Slam they were removed altogether and the opening faired over, with the rear of the bay cut away to clear the bomb's tail. Merlin 24 engines were fitted for better take-off performance, along with a strengthened undercarriage and stronger mainwheels later used on the Avro Lincoln.

How many were built is disputed: the reference encyclopaedia's variant listing and the article on the Grand Slam both give 32, while its operational narrative speaks of 33 B.Is modified in early 1945 for the larger weapon. Loaded with a Grand Slam the aircraft could barely manoeuvre and pilots were advised to leave the controls alone as far as possible.

Two airframes, HK541 and SW244, were further modified with a dorsal saddle tank of 1,200 imperial gallons behind a reshaped canopy to extend range. No. 1577 SD Flight tested them in India and Australia in 1945 with the Pacific in mind, but a full tank spoiled the handling and in-flight refuelling was preferred instead.