Yes, you can. There are a few ways.
You can fly your own bomber, and gun from it. You are essentially crewing the whole bomber. The bomber stays on autopilot while you gun, but you can still use the rudder to steer. You can jump between crew positions using the number keys, with 1 being the pilot's seat. The numbers vary from bomber to bomber based on the number of positions on the bomber. For example, on a B-17, 1 is the pilot's seat, 2 is the ball turret, and I believe 3 is the tail gun.
You can also join another person's bomber. This lets the pilot fly and drop the bombs, while you gun the entire time. To do this, you need to join their plane using the .join xxxx command. Type ".join playername" into the text buffer. The player you're joining will have to type ".accept yourname" to complete the join.
Always ask before you join another person's plane; don't just join random people. It pops up a message in the center of that player's screen, which has gotten me killed in the middle of a dogfight a few times, leading to a great deal of cursing.
Hope that answers your question.