Bendover,
from the following link....
http://www.spartacus.schoolnet.co.uk/2WWgrandslam.htm"In 1945 Barnes Wallis developed the 22,000lb Grand Slam. This bomb was so heavy it could only be carried by a specially adapted Avro Lancaster. The first one was dropped on Germany on 14th March, 1945."
I think B-29 adaptation was post-war. Here are a couple of other interesting notes on Tall Boy and Grand Slams from
http://www.bismarck-class.dk/tirpitz/miscellaneous/tallboy/tallboy.htmlTall Boy...
"These massive bombs designed by Dr. Barnes Wallis reached the speed of sound during descent being streamlined and equipped with angled fins that produced a rapid spin. Penetrating the ground before exploding they worked by setting off shock waves that would bring down nearby structures. The 5,443 kg (12,000 lb) "Tall Boy" dropped from 6,096m (20,000 feet) made a 24 meter (80 feet) deep crater 30 meter (100 feet) across and could go through 4.88 meter 16 feet of concrete. On 8-9 June, 1944 eight Lancaster bombers of No. 617 Squadron used the deep penetration "Tall Boy" bomb in an attack against the Saumur Rail Tunnel. The new weapon proved its worth but at the cost of losing 5 of the 8 bombers on this mission. Eventually 854 Tallboy bombs were used, the most note-worthy mission resulting in the destruction of the battleship Tirpitz (77 were dropped in three attacks)."
Grand Slam...
"The "Grand Slam" (Earthquake) bomb was of the same design as the Tallboy but larger and heavier weighing 9,979 kg (22,000 lb). The Grand Slam was first used on 14 March, 1945 when a force of Lancaster bombers led by Royal Air Force Squadron Leader C.C. Calder attacked the Bielefeld railway viaduct destroying two spans. In another attack against submarine pens (Bunker Valentin) near Bremen two Grand Slams pentrated 4,5 meters of reinforced concrete. 41 Grand Slam Bombs were dropped by the end of the war mainly against bridges and viaducts. The "Grand Slam" bomb was the biggest bomb used in WWII at all."
A bomb that reached the speed of sound....heh heh... There was a great documentary that showed fairly modern testing of fuel-air bombs to cause a miniature shock wave that rippled the surrounding landscape for hundreds of feet. It was like watching a small wave about 1-2ft high of earth running out from the blast center in a concentric ring. The shock wave exploded anti-tank mines in a test field as a proof of concept for the fuel-air bomb.