Donald Bailey was an civil servant in the British War Office who tinkered with model bridges as a hobby. He presented one such model to his chiefs, who saw some merit in the design and had construction started at a slow rate. A number were available by 1944 for D-Day, when production was ramped up. The US also licensed the design and started rapid construction for their own use. Bailey was later Knighted for his invention, which continues to be widely produced and used today.
The basic bridge consists of three main parts. The "floor" of the bridge consists of a number of 19 foot wide transoms that run across the bridge, with 10 foot long stringers running between them on the bottom, forming a square. The bridge's strength is provided by the panels on the sides, which are 10 foot long cross-braced rectangles. These are placed standing upright above the stringers, and clamps run from the stringers to the panels to hold them together. Ribands are placed on top of the completed structural frame, and wood is placed on top of the ribands to provide a roadbed.
Each unit constructed in this fashion creates a single 10 foot long section of bridge, with a 12 foot wide roadbed. After one section is complete it is typically pushed forward and another built behind it. The two are then connected together with pins pounded into holes in the corners of the panels.
For added strength several panels (and transoms) can be bolted on either side of the bridge, up to three. Another solution is to stack the panels vertically. With three panels across and two high, the Bailey Bridge can support tanks over a 200 foot span.
An astonishing feature of the Bailey Bridge is it's ability to be "launched" from one side of a gap. In this system the front most portion of the bridge is angled up into a launching nose and most of the bridge is left without the roadbed and ribands. The bridge is placed on rollers and simply pushed right across the gap, at which point the roller is removed (with the help of jacks) and the ribands and roadbed installed, along with any additional panels and transoms that might be needed.
Stories of Bailey Bridges being built and erected during WWII are legendary. In one instance a bridge was pushed over the Saar while until artillery and tank fire. When the enemy was finally cleared out the panels had holes in them and would not carry the weight of a tank. Replacing the panels would require the bridge to be "broken" in the middle. Instead they simply bolted an entirely new set of panels onto the bridge on top of the original set, a technique that later became a standard feature.