When Bombers become Strategic Bombers, they gain +40 hit points, +9 attack strength and +20 movement speed, as well as +150 Craft, but loses -10 attack speed. Their operational time is increased to 50 seconds (although fuel recharge time is increased also to 25 seconds).
The Strategic Bomber is even more powerful against buildings and has a greater range than Bombers. It has more health but it is still very weak to AA and enemy fighters.
Don't be tempted to use Strategic Bombers against enemy units as they are very ineffective so don't waste the fuels on them. The best tactic for using them is using five or more to attack a single target or different targets in a close proximity. This makes them less vulnerable to AA emplacements (there is more to shoot at and therefore less damage dealt) and allows multiple targets to be destroyed in one run.