This achievement is most easily obtained on the first mission, however, you will not be able to achieve it on your first run through as the plane you are flying, the Hurricane, does not carry any bombs.
Instead, on completing the mission you should unlock the Tomahawk. With this, set the difficulty setting to the lowest possible, and try and bomb one of the bombers that you come up against having 'flown along above the clouds' with Cliff.
It is important to set the difficulty on 'Rookie', as anything higher will result in you being torn to pieces by the bomber's rear gunners as you manoeuvre into position.
Also note, you will not destroy the bomber with one bomb. It will need two. What you can do, however, is shoot a few rounds into the bomber until a light trail of smoke appears from one of the engines. Then, one bomb will do the job.
Good luck, and if you want to do some multiplayer achievements, send me a friend request!