When the bombs leave the plane, they don't travel straight down. They accelerate downwards at 9.8 m/s/s, but they travel horizontally at the same speed as the plane. Look at the first photo. When the first (lowest) bombs were dropped, they would have been directly below the plane. The plane has since dropped a bunch more bombs, so some time has passed, and the plane has kept moving forward, yet the first bombs are still directly underneath the plane! The bombs are accelerating downwards, but they're also continuing to move along underneath the plane at the same speed as it! Assuming the plane keeps travelling with the same velocity, the only reason the bombs won't stay directly underneath the plane until they hit the ground is because the air resistance (drag) on them will slow them down. The consequence of this is that the plane has to drop its bombs well before it actually reaches its target, otherwise it will overshoot.

1-      2-       3-PLANE   PLANE    PLANEBOMB1   BOMB2    BOMB3        BOMB1    BOMB2                 BOMB1

The plane is moving from left to right across the textbox, and each bomb moves downwards, but also moves left to right with the plane! So in your game, when the bomb is dropped from the plane, make its velocity exactly equal to the planes, then each frame simply apply gravity and some air resistance to it.

If you want to test it, throw a rock straight out of a car window and watch where it lands. When it lands it will probably be closer to where the car is than where the car was when you threw it.