Britain’s Grand Slam was the Heaviest Bomb of WWII

The Grand Slam bomb, developed by Britain during World War II, is one of the largest conventional bombs ever used in combat, designed to break the sound barrier on its way to the ground and punch through meters of concrete and soil.

Designed to penetrate deep underground before detonating, its “earthquake bomb” design aimed to target and destroy heavily reinforced structures that couldn’t be damaged by conventional means.

At 22,000 lbs (11 tons), it was the single largest bomb ever created until the development of atomic weapons.



By 1943, the Second World War was starting to turn in favor of the Allies, but Germany had constructed vast fortified installations in occupied Europe that posed a significant challenge to Allied forces.

These structures, including U-boat pens, railway tunnels, V-2 rocket manufacturing facilities, and secret weapon sites, were protected by up to 8 meters of reinforced concrete and were immune to conventional bombs.

The inability to neutralize such structures highlighted the limitations of then-current aerial bombing tactics and the ordnance in use.

Valentin Sub Pen.
German concrete bunkers, such as this submarine pen in Valentin, could not be damaged with conventional weapons. Image by Olga Ernst CC BY-SA 4.0.

Standard bombs, no matter how large, were thin-walled to maximise the amount of explosives contained inside. Upon striking something like a reinforced concrete bunker, most of their explosive energy would travel upwards, causing minimal surface damage.

It became clear to the Allied forces that a new type of weapon was necessary, one that could reach beneath the surface and destroy these structures from below, a feat which conventional bombs could not achieve.

Into this context stepped Sir Barnes Wallis, already a respected figure within the British aviation industry for his pioneering work. He had already proven his innovative prowess with the design of the “bouncing bomb,” an ingenious device used in Operation Chastise, famously known as the Dambusters raid.

Bouncing bomb release.
A bouncing bomb being dropped by a Lancaster.

Wallis’s previous success made him a leading figure to address the challenge of the fortified structures. Fortunately, he had already given this exact problem some thought.

Instead of simply adding more explosives, which would do little to help, Wallis instead relied on the principle that energy travels more efficiently through a solid than through air.

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Using this, he needed a bomb that could explode underground near a target, sending immense shockwaves through the earth and into its foundations, like an earthquake.

Saumur Train tunnel collapsed.
A collapsed portion of the Saumur train tunnel, destroyed by “earthquake” bombs that penetrated through the hill above.

This would either demolish the structure, or crack and shift its foundations, rendering it unusable. This is why these bombs are often referred to as “earthquake bombs”, and “bombs designed to miss”.

Of course, while the theory was solid, actually creating such a device was a whole other problem. To do it, Wallis needed a object that could travel extremely fast and stay in one piece after it hit the ground.

Therefore it needed to be large, heavy, aerodynamic, contain large quantities of explosives, and have an armored casing to keep the explosives together during impact.

Initially Wallis drew up a massive bomb that weighed 22,000 lbs (10,000 kg) and needed to be dropped from 40,000 ft for optimum effect. With no Allied aircraft capable of such a task, Wallis then created a smaller 12,000 lb version, which would become known as the Tallboy.

After the Tallboy was successfully used against enemy targets, permission was given for the introduction of the 22,000 lb version, which was first known as “Tallboy Large”. Later, this would become Grand Slam.

Design Of The Grand Slam

The Grand Slam’s design was sleek and elongated, appearing almost visually identical to the Tallboy. This was not just for aerodynamic efficiency but also to aid in its penetration capabilities.

At the front of the bomb was a large casing, made of a chrome molybdenum steel casting. Inside, it contained 9,200 lbs (4,200 kg) of Torpex explosives, which is around 50% more powerful than TNT.

Ten Ton Tess bomb on a crane.
A Grand Slam bomb being moved by crane.

The casing was aerodynamically shaped and extremely strong, allowing the bomb to reach immense speeds during free fall, and ensuring the explosives remained intact after penetrating the ground. And, unlike the Tallboy, the Grand Slam was designed with concrete penetration in mind.

A sophisticated time-delayed fuse system was located at the rear of the explosive casing. This mechanism was calibrated to detect the sudden deceleration when the bomb stopped moving (i.e., when it had penetrated its target), and only then would it initiate the explosion.

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Behind the casing was a slender tail, topped with angled tail fins that caused the bomb to spin, improving accuracy. All in the Grand Slam measured 26.5 ft (8 meters) in length, and nearly 4 ft (1.2 meters) in width.

Grand Slam bomb casings.
Grand Slam bomb casings during manufacture.

Weighing in at 22,000 pounds (10,000 kg), the sheer mass of the Grand Slam was central to its effectiveness.

When dropped from a height of around 13,000 ft, the bomb would accelerate up to around 750 mph, breaking the sound barrier and becoming supersonic.

During descent the bomb essentially became a bullet with the weight of a school bus, carrying such immense energy that it would punch deep into the ground.

Grand Slam bomb casing stood up right.
The size of the Grand Slam is incredible – here, a factory employee stands next a casing.

After penetration, the 4.5-ton explosive charge would detonate. The energy would pass through the soil demolishing the target, or hitting it with a powerful “shove” that would shatter its foundations.

The shockwave could also create a ‘camouflet’, a large underground cavity. The ground above this cavity would collapse, and any structure on or near the surface would be severely damaged.

Ashley Walks Bombing Range Grandslam crater.
A crater left by a Grand Slam bomb on the Ashley Walks Bombing Range in the UK.

Deployment Of The Grand Slam

The Grand Slam was first tested on the Ashley Walk Bombing Range in the New Forest on March 13, 1945. Due to the hurried nature of the project and a lack of spares, a live bomb was used for the test drop.

After a successful test, Grand Slams were being dropped on targets in Europe days later.

The Grand Slam’s sheer size posed logistical and operational challenges. Weighing 22,000 pounds, it required a robust delivery platform. The Avro Lancaster, an iconic symbol of British aerial warfare and the most powerful bomber in the RAF’s arsenal, was chosen for this purpose.

First test of the Grand Slam bomb.
The first test of the Grand Slam, photographed as it descends above Sandy Balls in the New Forest, UK.

However, while the Lancaster could carry the smaller Tallboy with minimal changes, it required extensive modifications to carry such a heavy ordnance.

The Lancaster B.I (Special) variant emerged as a result, modified specifically to handle the Grand Slam’s weight and dimensions.

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They had their bomb bay doors, electrical systems, armor, fuel tanks, most of the weapons, tools, ladders and more removed to lighten them. In addition, more powerful Merlin variants were fitted.

Lancaster B.I (Special)
Lancaster B.I (Special) carrying a Grand Slam. Note the enormous size of the bomb compared to the aircraft.

Even with all these changes the Lancaster B.I (Special) was extremely sluggish and unable to manoeuvre.

On March 14, 1945, the Grand Slam was used in combat for the first time. The target was the Bielefeld viaduct in Germany, a crucial railway junction that facilitated the movement of German troops and supplies.

The viaduct had previously been attacked by thousands of tons of bombs, but the Grand Slams brought a huge section down on their first use.

Bielefeld viaduct, destroyed.
The Bielefeld viaduct following the successful attack from Grand Slam and Tallboy bombs.

Following the success at Bielefeld, the Grand Slam was deployed in several other critical missions, such as against the Arnsberg viaduct, bringing it down on the second attempt.

Aiming to paralyze Germany’s logistic and operational capabilities, Grand Slams were often used against railway lines and bridges.

Targets also included submarine pens with incredibly thick concrete walls and roofs. Despite being designed before their existence had become known, Wallis’ Tallboy and Grand Slam bombs were the only Allied ordnance capable of dealing with them.

The destroyed Arnsberg Viaduct.
Arnsberg Viaduct after Grand Slam attacks. Note the enormous craters around the viaduct, created by Grand Slams and Tall Boys.

On March 27, Lancasters armed with Grand Slams attacked German submarine pens in Valentin, France. This was a huge concrete structure that protected the production of submarines. Two Grand Slams penetrated the the thick reinforced roof, bringing down 1,000 tons of the structure with them.

In just their first month of use, Grand Slams had destroyed four major targets.

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The deployment of the Grand Slam was not without its challenges. Given its size and the specific tactical requirements, each mission had to be meticulously planned.

The hole in the roof of the Valentin submarine pen.
The hole left by a Grand Slam bomb that pierced the reinforced concrete roof of the German submarine pens in Valentin.

Factors like the altitude from which the bomb should be dropped, the angle of descent, and ensuring the safety of the deploying aircraft were all critical considerations.

Moreover, the limited number of modified Lancasters and Grand Slam bombs meant that each mission’s success was imperative.

Other Uses

When the war ended, only 41 Grand Slams had been used in anger, but they had claimed a disproportionally large number of important, and extremely hardened targets.

They were eyed for use in the Pacific, dropped by B-29 Superfortresses against dug in Japanese defenders, but this did not eventualize.

Still, they were tested on B-29s, with one even being modified to carry two (one under each wing). In American service, they were designated M110.

Explosion on the Arnsberg viaduct.
The explosion of a Grand Slam, witnessed from the air.

The B-29 was much better suited to carry this bomb, able to fly higher and further with it slung underneath.

Grand Slams would serve as the largest bomb used during the entire war, only beaten by atomic weapons in its final weeks.

B-29 with M110 bomb.
This B-29 has been modified to carry a Grand Slam. The bomb bay doors had to be cut open as the bomb could not fit inside the bay.

But even after this, the United States was still interested in penetration bombs, creating an enlarged version that was twice as heavy as the Grand Slam named the T-12 Cloudmaker.

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The T-12 weighed a staggering 44,000 lbs (20,000 kg), and could only be carried by the B-36 Peacemaker, although a B-29 also carried it during tests.