Basically this is how it works:
In single player, a special program is required to fly AI aircraft, such as the B-17.
Online in a multiplayer game, as long as external views are on, and the host has loaded the AI planes into the mission plane lists, they are flyable. No cockpits, though.
Most AI fighters are worthless without being able to aim, but the bombers, like the B-17 and the Pe-8 (which carries some big-ass bombs

), are just plain fun to fly. The sea-planes are a hoot, too
Here's a link to the Cossacks homepage:
http://www.cossacks.com/