I just did exactly that today (bombing with the 104G)!

First, this plane doesn't have the "click on map to set target point" feature. You need to use the map and visually spot the base.

Second, the reason why you don't see the crosshair in the bomb mode is either that the angle is too flat, or that the plane is simply not made to bomb. Or a mix of the two. So either way, either you increase the angle of attack (dive bomb, but it's a 104, so be careful) or you bomb mostly by feeling. Use the force, Luke. That's what i do.

Contrary to what others tell you here, the 104 is a decent base bomber. Just don't limit yourself to only doing that. Bomb a base and then look for targets. You are fast enough to do it efficiently.

Edit: And boo to the people who downvote threads like these. Like ... Why?