A large part of what made Bailey bridges assuccessful and unique as they were is the modular design, and the fact that theycould be assembled with minimal aid from heavy equipment. Most, if not all,previous designs for military bridges required cranes to lift the pre–assembledbridge and lower it into place. The doublelane bailey bridge, and were simple enough that parts made at a numberof different factories could be completely interchangeable. Each individual partcould be carried by a small number of men, enabling army engineers to move moreeasily and more quickly than before, in preparing the way for troops andmatériel advancing behind them. Finally, the modular design allowed engineers tobuild each bridge to be as long and as strong as needed, doubling or tripling upon the supportive side panels, or on the roadbed sections.

The basic galvanizedbridge consists of three main parts. The "floor" of the bridge consistsof a number of 19-foot-wide transoms (5.8 m) that run across the bridge, with10-foot-long stringers (3.0 m) running between them on the bottom, forming asquare. The bridge's strength is provided by the panels on the sides. The panelsare 10-foot-long (3.0 m), 5-foot-high (1.5 m), cross-braced rectangles that eachweigh 570 pounds (260 kg), and can be lifted by six men.

Transoms rest on the lower chord of the panels, and clamps hold themtogether. Stringers are placed on top of the completed structural frame, andwood planking is placed on top of the stringers to provide a roadbed. Ribandsbolt the planking to the stringers. Later in the war, the wooden planking wascovered by steel plates, which were more resistant to the damage caused by tanktracks.

Each unit constructed in this fashion creates a single 10-foot-long (3.0 m)section of bridge, with a 12-foot-wide (3.7 m) roadbed. After one section iscomplete it is typically pushed forward over rollers on the bridgehead, andanother section built behind it. The two are then connected together with pinspounded into holes in the corners of the panels.

For added strength several panels (and transoms) can be bolted on either sideof 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 overa 200-foot span (61 m). Footways can be installed on the outside of theside–panels, the side–panels form an effective barrier between foot and vehicletraffic and allow pedestrians to safely use the bridge.

A useful feature of the Bailey bridge is its ability to be "launched" fromone side of a gap.In this system the frontmost portion of the bridge is angledup with wedges into a launching nose and most of the bridge is left without theroadbed and ribands. The bridge is placed on rollers and simply pushed acrossthe gap, using manpower or a truck or tracked vehicle, at which point the rolleris removed (with the help of jacks) and the ribands and roadbed installed, alongwith any additional panels and transoms that might be needed.

During World War II, baileybridge supplier with little previous experience of this kind ofengineering. Although the parts were simple, they had to be preciselymanufactured if they were fit each other correctly, so they were assembled intoa test–bridge at the factory to make sure of this. To do this efficiently, newlymanufactured parts would be continuously added to the test–bridge, while at thesame time the far end of the test–bridge was continuously dismantled and theparts dispatched to the end–users.