Barnes Wallis developed the Tallboy bomb in 1944. It weighed 12,000lb and had to be dropped from at least 20,000 feet. Also known as the earthquake bomb it was successfully used against V1 Flying Bomb launch sites and in the sinking of Germany's giant battleship, Tirpitz, on 12th November, 1944.
(1) W. J. Lawrence wrote about the Tallboy bomb in his book, No 5 Bomber Group (1951)
It was an extraordinary weapon, an apparent contradiction in terms, since it had at one and the same time the explosive force of a large high-capacity blast bomb and the penetrating power of an armour-piercing bomb. On the ground it was capable of displacing a million cubic feet of earth and made a crater which it would have taken 5,000 tons of earth to fill. It was ballistically perfect and in consequence had a very high terminal velocity, variously estimated at 3,600 and 3,700 feet a second, which was, of course, a good deal faster than sound so that, as with the V-2 rocket, the noise of its fall would be heard after that of the explosion.
(2) Arthur Harris, Bomber Command (1947)
We already had Wallis's 12,000 Ib. medium capacity bomb, which was capable of breaking through the roof of a railway tunnel or a very thick concrete roof, and when the success of this bomb was proved Wallis designed a yet more powerful weapon, the 22,000 Ib. bomb, the most destructive missile in the history of warfare until the invention of the atom bomb. This 22,000 Ib. Bomb did not reach us before the spring of 1945, when we used it with great effect against viaducts or railways leading to the Ruhr and also against several U-boat shelters.