How do bombard other units?

The manual says I can do it, but no button on the interface. Just the B button, which does not work when I am attacking other units or cities whether I am a catapult, cannon, frigate or a destroyer.

You can only bombard cities with any land-based units. Bombers and fighters can bombard units, only to 50% of their health.

The manual is confusing.

In Civ III you could do this , but now you can only bombard to take away fortification bonus and city defences.

Once these reach zero, the bombard option is invalid.