I have written an article for a Dutch air war magazine about the development of the MC bombs and have translated it in English as best as possible. If someone know when the first 250 lb. version was used, please let me know. I think around October 1943.
Development of the MC bomb began in early December 1940 when, during a meeting attended by the Deputy Chief of Air Staff (Air Vice-Marshal Arthur Harris), it was determined that the General Purpose (GP) Bombs then in use were inadequate. Intelligence reports had shown that due to the charge ratio of only 17-33% (weight of explosive compared to the total weight of the bomb), the effect of the GP bombs was very limited. The meeting decided that the recently introduced High Capacity (HC) Bomb, which was intended to provide blast effect, was not always applicable and the replacement for the streamlined GP bomb should be a British version of the German SC Bomb. So a bomb with a cylindrical middle section, a tapered nose section and a tail that was secured with clamps or bolts. The charge ratio had to be as high as possible, with the bomb body being strong enough to withstand impact into paved roads and all types of infrastructure (except reinforced concrete), including deep penetration into multi-storey buildings. The demands were passed on to the Ordnance Board the same month and work on the first bombs began immediately. It became the 500 lb. MC bomb, the initial manufacture and testing of which presented some difficulties.
Three designs were ultimately produced; a welded type (Mk I), a forged drawn tube type (Mk II) and a cast type (Mk III). The welded type could be produced in large quantities quickly, but the varying levels of welding technology at factories across the country did not always guarantee that the bombs would remain intact on impact, especially when a long-delay fuse was used. The forged type had the best results, but took longer to produce. The charge ratio of the Mk I and Mk II was approximately 50%. The cast type had a heavier body, so the charge ratio was approximately 42%. During the Second World War, about 15 different Marks of the 500 lb. version was created on the drawing board, with 12 models produced in larger numbers. The 500 lb. Mk I to IV only had a British suspension lug. All subsequent models were also equipped with two American suspension lugs, so that they could also be transported by aircraft with American suspension devices. The 500 lb. Mk V was a Mk III that had an incorrect center of gravity due to a manufacturing defect. Instead of scrapping the 6,000 pieces, they were fitted with a longer tail, which solved the problem. Forged bombs were mainly used against buildings with heavy beams and the welded bombs against targets where direct detonation on impact was necessary. When a long delay fuze was used, the forged bomb was preferred. If not available, a die-cast bomb could be used. With the arrival of the heavy four-engine bombers, the Air Staff indicated that a 1000 lb. MC was desired. From February 1943, approximately 18,000 units of this 1000-pounder version, in cast and forged versions, were produced per month, but it was not until mid-April of that year that this bomb was found on the Bomber Command lists. The 4000 lb. MC was developed hastily in the spring of 1943 to replace the 4000 lb. GP version and was intended for bomb attacks from low altitude. The bomb only had a British suspension lug, but American suspension lugs could be fitted and it was only fitted with a fuze in the rear. The charge ratio was 55%. When the bomb became available, the Commander-in-Chief of Bomber Command (Air Chief Marshal Sir Arthur Harris) was reluctant to use his valuable four-engine bombers in low-altitude attacks and the bomb was carried by the heavy bombers in normal bomb attacks from high altitude, such as during the bomb attack on the dike at Westkapelle (The Netherlands) on October 3, 1944, when 237 of these MC bombs were dropped. As far as I know, the first 4000 lb. MC was dropped during a bomb attack on La Spezia, Italy, on the night of April 13/14, 1943.
Mosquitos used the bomb with an 11 second delay fuze effectively in low-level attacks, where the bomb should not be confused with the HC bomb of 4000 lb. (Cookie) used by heavy bombers and Mosquitos in high-level attacks. Finally, probably in October 1943, the 250 lb. came from the factory consisting of a cast body and a charge ratio of 42.44%. The Mk I only had a British suspension lug and the Mk II also had American suspension lugs. This bomb was not of interest to Bomber Command and did not include it in its arsenal. This MC bomb was used by fighter-bombers such as the Spitfire, Hurricane, Mustang, etc. The so-called Deep Penetration Bombs of 12,000 lb. (Tallboy) and 22000 lb. (Grand Slam) were also covered by the MC bombs in the RAF in connection with their charge ratio, but were only used against special targets where penetration into sub-soil strata was usually required.
250 lb. Mk I = cast body
250 lb. Mk II = cast body + US suspension lugs
500 lb. Mk I = welded body
500 lb. Mk II = forged body with welded bottom
500 lb. Mk III = cast body
500 lb. Mk IV = cast body with the length of the GP bomb Mk IV
500 lb. Mk V = Mk III with longer tail due to production error
500 lb. Mk VI = Mk I + US suspension lugs
500 lb. Mk VII = Mk II + US suspension lugs
500 lb. Mk VIII = Mk III + US suspension lugs
500 lb. Mk IX = Mk IV + US suspension lugs
500 lb. Mk X = = forged body with solid nose
500 lb. Mk XI = Mk VII with solid nose
500 lb. Mk XII = Mk VII with better welded bottom
500 lb. Mk XIII = welded body, solid bottom
1000 lb. Mk I = cast body
1000 lb. Mk II = cast body, + stronger US suspension lugs
1000 lb. Mk III = forged body with solid nose
4000 lb. Mk I = welded body, nose internally reinforced with plates
4000 lb. Mk II = welded body, nose internally reinforced with a solid piece of steel, nose plug welded on
12000 lb. Mk I = cast body, no suspension lugs
12000 lb. Mk II = Mk I produced in the USA
22000 lb. Mk I = cast body, no suspension lugs
22000 lb. Mk II = Mk I produced in the USA, no suspension lugs
