Really, loads and loads of bombers is something you can't go wrong with. There have actually been games where players have built nothing but one soldier and bombers on top of it and won; the lethal bombardment ability is extremely effective and with enough bombers it's way overpowered. A single stack of the things in the dozens can rule a continent with ease.
AA units don't stand a chance against enough bombers. If your economy can take the loss, just bomb the crud out of the Flak guns. AI never thinks to build loads of them.