Dude,
in other posts is the info about bombsights. However to recap...
In a b17 or B26 make sure you are flying level in the autopilot, (press the X key to get a yellow light showing on the dashboard).
press F6, (if your in a gunner seat you need to select the pilot position by pressing 1 first).
to look in the bombsight press the 0 (zero), key on the number pad. You should see the crosshairs of bombsight. Note that up to v0.46 the engine noise, (and pings from attacking aircraft), will cut out and the bomb drop sound will be heard after you look up from the sight, (this depends on how much zoom you are using).
You can toggle the zoom on the bombsight by pressing the Z key, (this also works for gunners and external views).
You can also gradually zoom in and out by pressing the [ or ] keys.
Align the cross hairs onto the target by rolling the plane left or right. The plane will automatically level when you recenter the joystick.
Press the B key, (or button 2 on your joystick), to release the bombs. Note that up to v0.46 the bombbay doors open automatically. Keep adjusting left and right dropping bombs untill you pass the target and have to circle back. Repeat as necessary or untill out of bombs =)