When airborne with the Grand Slam, a Special could barely manoeuvre, and pilots were advised to refrain from minor adjustments of the flying controls, allowing the aircraft to wallow.

Grand Slams and Tallboys were capable of causing damage that smaller bombs could not, accelerating the collapse of German resistance and avoiding mass civilian casualties.

The Tallboy was discovered to be too small to destroy concrete buildings with direct hits; only near misses which opened a camouflet under the foundations were found to be effective.

In September 1944 the German V-2 rocket bombardment of London began, which had been expected by the British since Operation Hydra (the attack on the Peenemünde research centre on the night of 17/18 August 1943), and Grand Slams were considered as a weapon to be used on V-2 launch sites.

The British order was cancelled on 4 October, leaving only US production, but RAF representatives in the US were able to persuade the Americans to build fifty bomb casings by the end of the year, to be filled in Britain.

During the Battle of the Bulge (16 December 1944 – 25 January 1945) any complacency about an imminent end to the war was confounded; the Grand Slam production order was revised again for 25 from British sources and 200 from the US.

[8] An expanded version of the Tallboy design was not feasible and the English Steel Corporation of Sheffield began work on "large boilers" in late 1944.

The commander of 5 Group, Air Vice-Marshal Ralph Cochrane ordered that the Identification friend or foe (IFF) be got rid of in favour of the VHF Bomber Fixer.

Despite bombing by day, navigation equipment was needed because of the north-west European climate; GEE and LORAN were to be carried along with high-altitude and low altitude radio altimeters.

As Messerschmitt Me 262 jet fighters were operating in northern Germany, an Automatic Gun-Laying Turret (AGLT; also, Village Inn or Z Equipment) was considered.

The commander of 617 Squadron, John "Johnny" Fauquier, ordered that if returning with a bomb, the Special would divert from Woodhall Spa and use the longer runway at RAF Carnaby near the coast at Bridlington in East Yorkshire.

[14] The centre of gravity when loaded with a Tallboy or Grand Slam was within the aircraft design limits, provided the rear turret was retained.

Particular attention was to be paid to the skin of the wings and rear fuselage when checking the airframe and the Special would take off only from smooth runways and always with experienced pilots.

[15] Fuel was to be used from inboard tanks to outboard ones to relieve the stresses on the wings, When the store is carried the aircraft is to be very carefully handled and is restricted to gentle manoeuvres only.

[15] A Ransomes & Rapier crane at the A&AEE could lift the Grand Slam when on a concrete base and the Type H trolley was acceptable when limited to 10 mph (16 km/h) with its tyres at 80 PSI.

[15] The bomb dump at Woodhall Spa had been limited to forty Tallboys but room was made to accommodate ten Grand Slams by removing ordnance not in use.

The aim of the crew was accurate for distance and observers saw the black and white painted bomb rotate as it fell, about 100 ft (30 m) off line to starboard.

[22] The Lancasters of 617 Squadron flew around Bremen then found that there was cloud on the north side of Bielefeld forcing PD112 S with the Grand Slam to attack from the south after resetting the SABS, followed by the photographic Mosquito.

A second wave bombed fairly accurately but one Grand Slam fell 50 yd (46 m) off line due to a SABS fault.

[32] Twenty Lancasters of 617 Squadron, two carrying Grand Slams and the rest Tallboys, flew in good weather with little cloud to Bremen to attack the double-tracked railway bridge at Arbergen which crossed the Weser near Nienburg.

Another five fighter squadrons were provided to escort a day raid by 1 and 8 Groups on an oil refinery at Bremen; other Main Force aircraft attacked targets in Münster and Rheine.

Later analysis found that a Tallboy had landed on the bank at the south end of the bridge about 100 ft (30 m) from the abutment and the shock of the explosion shifted it, making the span fall into the river.

A Tallboy hit the north end of the bridge, the damage being swiftly repaired; another 250 yd (230 m) to the south which was filled in and new track laid.

The Grand Slams penetrated 8 ft (2.4 m) into the concrete causing about 1,000 long tons (1,000 t) of the roof to fall in bringing down two moveable cranes.

[45] Seventeen Lancasters from 617 Squadron were dispatched to bomb the Finkenwerder U-boat pens in Hamburg, two with Grand Slams and the rest with Tallboys.

The six Tallboys had penetrated the roof part way, hit girders and exploded, blowing holes through the ceiling and depositing hundreds of tons of concrete into the pens.

[55] By the end of the war, 41 Grand Slams had been dropped on operations although in 2004, Stephen Flower wrote of 42 bombs, a figure he repeated in his 2013 publication.

[57]In 2013, Stephen Flower wrote that when dropped on buildings fortified with reinforced concrete the Grand Slam tended to break up on impact or go off too soon.

Flower wrote that it was not surprising that the Grand Slam was limited in effect when dropped on concrete, when it was designed to fall on earth and then explode under the foundations of buildings, creating a camouflet for the structure to subside into.

The T-12 Cloudmaker is an American-made variant of the Grand Slam and an example is displayed at the Air Force Armament Museum in the United States.