Bunkers are special defensive buildings that are built by Engineers (and noobs) rather than deployed by vehicular transports. To build a bunker, select a regiment of Engineers and click on the Build icon in the command panel. Choose the type of bunker you wish to construct, and then place it as you would if you were unpacking a standard building. Your Engineers will then rush over and build the bunker at the location you specify.
Source: Axis and Allies Official Manual
