It is a tribute to the design of the Avro Lancaster that the Mk I remained in service as the main version of the aircraft from the time of its introduction to the end of the Second World War. Of the 7,377 Lancasters produced, 3425 were Mk Is. Another 3,469 were Mk IIIs or Mk Xs, which were essentially Mk Is powered by the American-produced Packard Merlin engine.
The Mk I closely resembled the second prototype. The engine used was the 1,280 hp Merlin XX, which was later replaced by the Merlin 22 and then the more powerful Merlin 24. Defensive armament was provided by four Frazer-Nash turrets, armed with .303 Browning machine guns – two in the nose mounted FN5, two in the FN50 mid-upper turret, four in the FN20A tail turret and one in a FN64 ventral turret. The ventral turret was phased out during 1942 after it proved to be of little use in action.
The standard crew of a Lancaster was seven strong, consisting of a pilot, navigator, flight engineer, wireless operation, two gunners and the bomb aimer, who also operated the front turret.
The Lancaster could carry a heavy bomb load, but more importantly its 33 foot bomb bay was a single undivided unit, and could carry the increasingly large bombs used by Bomber Command during the war – the standard Lancaster Mk I was eventually able to carry the 12,000lb Tallboy bomb within the bay. The only real change required to carry the larger bombers was the use of bulged bomb days.
Improvements in technology did result in some changes to the appearance of the Lancaster. 1943 saw the introduction of downward looking H2S radar, a navigational aid that could identify built-up areas. The aerials connected to H2S were contained in a pear-shaped radome below the rear fuselage. A more short lived addition was backwards pointing radar, codenamed Monica, designed to help detect incoming night fighters. This had to be abandoned after it was discovered that the Germans were homing in on the radar signals.
B. Mk I/III Special (Dambuster)
Two Special Lancasters were developed during the war. The first Special was designed for 617 Squadron, to carry Barnes Wallis’s “bouncing bomb”. This bomb needed special fittings to carry it, and had to be spinning fast when it was dropped. Nineteen Lancasters had their bomb bay doors removed, and the required equipment added. The upper-mid turret was also removed. During the Dams raid (16/17 May 1943) eight of the nineteen Specials were lost, but two of the Ruhr Dams were destroyed. After the raid the surviving aircraft were restored to normal Mk I/II standard.
B. Mk I Special (Gram Slam)
The second Special Lancaster was developed to carry another Barnes Wallis bomb, this time the 22,000lb Grand Slam. This was a streamlined bomb designed to be dropped from as high an altitude as possible. It was designed to dig its way into the group, and cause an earth quake. The Lancaster could carry it, but only at the cost of the removal of both the nose and mid-upper turret, while two guns were removed from the rear turret. As with the Dambuster Specials, the bomb bay doors had to be removed, and the gigantic bomb dangled below the Lancaster, giving the aircraft a rather peculiar appearance (almost like a gigantic torpedo bomber!).
The Grand Slam equipped Lancasters had a range of 1,650 miles, and could only reach 17,000 ft, but even from this lower altitude the 41 Gram Slams that were dropped in the last months of the war lived up to Wallis’s expectations, and almost inevitably destroyed their targets.
B. Mk I (FE) – Far East
The Mk I (FE) was an interim design for use in the Pacific. It would have carried an extra 400 gallons of fuel in the bomb bay, and was equipped with the best navigation equipment available at the time. The Mk I (FE) was intended to make up the British component in the forces being built up in preparation for the invasion of Japan, but the war in the Pacific ended before any reached the area.