Easily obtainable in the first mission. When Cliff points out the enemy bombers to you, head towards them, maintaining your height. When you are almost directly above them, dive, dive, dive!
I'm not quite sure what 'full speed' is, but if the edges of your screen are blurred, and you kill one of the bombers on the way down, you stand a 99% chance of unlocking this achievement!
Could well be the first achievement that you unlock; 'twas mine!