I tried to do that once using the fire command, but could'nt get it to work. You can fake it using the camcreate, setdir, getdir, and setvelocity commands. Here's an example-
bomb1 = "LaserGuidedBomb" camcreate getpos hog1; bomb1 setdir getdir hog1; bomb1 setvelocity [50,0,-10]
The setvelocity command has to be tweaked depending on the speed and direction of the plane. The above example was for an A10 flying east at limited speed.