Just a little tip for those getting the Flying Fortress achievement: When it says you need to bomb an "enemy" city with a B-17 to get the achievement it actually means an enemy Civ's city. City-States don't count towards the achievement.
Also I found the controls to operate a bomber a bit unwieldy. In this version of Civ V you cannot bomb a city unless you have Line-of-Sight. i.e. if the city isn't revealed by the Fog of War, you can't bomb it, which means you will need a Forward Observer. Also when the bomber unit is selected it doesn't show your operational range, but when you use the Air Strike mission button, it does!
Mind you, once you've got a B-17 bombing a city, it really takes down its defenses like there's no tomorrow!