When an object is dropped/released by any moving vehicle. Then initial velocity of the object is same as the moving vehicle.
When the bomb is dropped from Plane the plane which is moving horizontally. So, the bomb will have same initial velocity as that of plane along horizontal direction.
The situation is shown in the diagram below. Let a fighter plane, when it be plane at position P, drops a bomb to hit a target T.
Let the target is seen at an angle θ with horizontal.
Important point: Angle is with respect to target. As seen by observer in the plane, motion of the bomb will be vertically downward below the plane.