that was heavily armored, but agile enough  to climb. From 1941 to 1945 thousands  

of what The New York Times called the  "world's deadliest tanks" were made. 

But there was a problem: the Churchill tank  weighed at least 40 tons, 36 metric tons.  

Imagine 20 cars. That would cause a normal bridge  

to buckle without a way to cross water,  the Churchill tank was basically useless. 

American tanks had the same problem. And as the  war waged on, the allies needed new bridges and  

replacement ones. Bombed out bridges like this one  in Germany needed a sturdier fix than a gangway.  

The solution to both of these problems was a new  type of bridge that improved on old breakthroughs  

and also represented a model maker's way of  thinking about how a bridge could win a war. 

Here's a bridge being built in China in 2020. Now  imagine no heavy machinery, no custom design, no  

materials heavier than a person could carry, and  no reliable access to the other side of the river.  

And half the time, you had to build it at night. That's the challenge of a wartime bridge.  

Donald Bailey was an engineer at the British  Experimental Bridging Establishment in 1940,  

and he needed a way to make  the standard temporary bridge,  

the Inglis bridge work for a new generation  of heavy tanks. When the Inglis bridge buckled  

under test weights, Bailey took an envelope  out and sketched an idea for a new bridge. 

He had made models since he was a kid and used  them to help test his design. He needed something  

adjustable to a range of gaps. It could  work with a pontoon floating in a river  

as well as with land. It had to be  made of something other than aluminum  

because airplanes needed that. It could be made  easily by manufacturers. No confusing parts,  

but most importantly, everything had to fit in a  small service truck and be lifted by six soldiers. 

The design is strong and more flexible  than earlier ones like the Inglis Bridge  

or this small box girder bridge that  was also used. The panel was smaller  

than a box girder, lighter and easier to stack. This is the key to his design. I'll show you. 

It's the real breakthrough ingredient. This  reinforcement makes a bridge that's really strong,  

but at just five hundred and seventy pounds  or two hundred and fifty kilograms, six  

people could lift it. Other designs like the  Inglis bridge, twisted under heavy weight. 

The Bailey Bridge didn't. You can see how easy it is to move in  

this video made shortly after the war. Panels were  joined by pins driven in. After that, soldiers  

laid a transom across. I used glue to join  mine, but clamps made the process even easier. 

But it is the next step,  before finishing the bridge,  

that made the Bailey Bridge even more special. See these men pushing? You could roll a Bailey  

Bridge across a gap—important when you  couldn't get to the other side during a war. 

I can kind of show you with  mine and this roller right here. 

Rollers to slide the bridge alone and  a link to make the front of the bridge  

slightly tilt up by making the bottom longer than  the top. You can see it here in this Army manual. 

The bridge is pushed along the rollers with  more weight, usually more panels in the back  

to keep it from tipping over. Sometimes  they drive on it as a counterweight too.  

In the front, this link makes the nose tilt up. The snoot didn't droop. 

That helps keep it from sinking into  the gap as it gets pushed across. 

Adding a second or even third level was easy,  thanks to the Bailey Bridge's modular design.  

These bridges quickly became an  indispensable tactical trend. 

By 1943, wartime manuals were  sandwiching crucial Bailey Bridge facts  

in between information about German  guns and warnings about dangerous fish. 

The Allies built twenty five hundred Bailey  Bridges in Italy and fifteen hundred in  

northwest Europe, as well as some in other  theaters, allowing those heavy Churchill tanks  

and the American equivalents to  move over land and water alike. 

General Dwight D. Eisenhower said,  "along with radar and the heavy bomber,  

it was one of the three most important engineering  and technological developments of World War II." 

Now, labeling the Bailey Bridge as  the reason the Allies won the war,  

it's impossible to say. However, a model maker's  breakthrough gave them a tool that was flexible,  

fast and strong in a war that  needed all three of those things. 

The modular design presaged a more  flexible, nimble type of construction. 

And the Bailey Bridge did allow them to cross the  rivers in Italy and push a bridge over the Rhine.  

They could use a Bailey Bridge, playing accordion  

over the water. And  

they could cross and play piano amidst the ruins.