Although the six-ton Tallboy bomb had been proven in action on numerous occasions, the original concept of Barnes Wallis had been for a weapon weighing ten tons. With ever increasing improvements to the Lancaster, Wallis' dream could now become a reality. This larger weapon, christened 'Grandslam' was identical in shape and construction to 'Tallboy', but was 10,000 Lbs heavier and at twenty-six and a half feet, was four foot five inches longer. It featured the same offset tail fins and revolved at a maximum 60rpm, dropped from 10,000 feet Grandslams regularly went super-sonic. The weapon was suspended from the same linked chain as Tallboy, only extended to accommodate Grandslams considerably greater girth. A batch of 32 Lancasters was completed to carry Grandslam, these aircraft being given the designation B 1 Special . Reminisent of the original Type 464 (P), each one had it’s bomb bay doors removed and the bay fitted with aerodynamic fairings each end, they also featured further improved Merlin engines. To offset the huge weight of the bomb and to increase range, certain sacrifices were made. The mid upper and nose turrets were removed, along with the Elsan lavatory and radio equipment. Even the navigators chair was removed and replaced with a wicker one. With the ever decreasing threat from the Luftwaffe and the ready availability of fighter cover, missions would be all daytime Ops. To this end the aircraft were given a daytime camouflage scheme of light earth and green upper surfaces over mid grey undersides. The B 1 Specials were given the new squadron code of YZ . Fauquier flew PB997 / YZ – E on March 5th and found it to be fast and responsive. With the Bielefeld Viaduct still seen as a priority target, a fourth strike was attempted on March 9th, but incredibly the weather once again thwarted the squadrons efforts and crews were ordered back to base. However this presented the ideal opportunity to try Grandslam operationally, so another attack was planned using the new weapon, supplemented by Tallboys. On March 13th, Fauquier in PD119 / YZ – J and Jock Calder in PD112 / YZ – S, took off carrying the only two Grandslams in existence. With the mid-upper and front turrets removed only the rear gunner remained, whilst the omission of the radio sets meant the wireless operator also stayed behind leaving a five man crew. Unbelievably thick cloud again came to the rescue of the viaduct and crews turned for home without bombing. Eager to see the target put out of action, No 5 Group hastily scheduled another trip the following day. At 13:00 hrs on the 14th all was ready, however as Fauquier went through his pre-flight checks and started engines he found he had a problem. A broken connecting rod in his starboard inner engine meant YZ – J was going nowhere. This left Calder to potentially make history by being the first to drop the largest bomb in the world. Refusing to give up the mission, Fauquier climbed from his aircraft and sprinted across the tarmac, his intention was to relieve Calder and commandeer his aircraft. However, Calder seeing Fauquier approaching immediately guessed what was going on. Mischievously turning a blind eye, he opened the throttles and roared off down the runway, leaving Fauquier planted firmly on the ground. The attacking force consisted of 12 Lancaster from No 9 Sq detailed to attack the Arnsberg Viaduct and 20 from No 617 Sq attacking Bielefeld. Although there was some patchy cloud, conditions were far better than 24 hrs earlier. Calder was determined not to have any reason to abort his drop again and after a short run in he released his Grandslam from just below 12,000 feet. The Lancaster, relieved of its ten ton load, rocketed 500 ft straight up, having regained his composure and control of his aircraft, Calder brought the bomber round to view the result.