During WW2 many planes, including almost all bombers on all sides, but also many twin engined fighter bombers, recon aircraft, dive bombers and more had defensive machine guns in turrets or other positions to defend against enemy fighters intercepting the aircraft.
To me they seem very ineffective. The plane and the enemy fighter will both fly at really high speeds, making hitting the enemy really difficult as well as giving only a small window of opportunity to shoot at it. Furthermore, most planes did not have the massive defense armament of a B17 but rather just 1 to 3 machine guns.
So how effective were they really? Were they a waste of production or really reducing bomber losses?
The answer to this, to the best of my knowledge, is "we don't know". I've never seen a comprehensively researched attempt to establish how many Luftwaffe attacking fighters were lost to the defensive armament as a whole during the war. The best data I'm aware of is for individual engagements. What I can tell you though is that how many kills the gunners scored is not the only reason a bomber has a defensive armament.
Engaging heavy bombers with fighters has not one but two primary purposes. The first is to shoot down the enemy bomber of course but the second is to disrupt it. Make it unable to complete its mission or miss its target. The reason strategic bombers, especially later models, had significant defensive armament is not necessarily to make it kill more enemy fighters but to make it a more difficult target. The more fire coming out of a bomber formation, the more careful a fighter pilot has to be in engaging it. Effectively making it harder for the fighter pilot to complete his mission of downing or disrupting the enemy bomber.
Now, bomber crews did claim quite a few kills on some raids. During the somewhat infamous Second Raid on Schweinfurt for example the three bomber wings involved claimed 186 downed luftwaffe fighters but lost a staggering 77 B-17's with a further 121 damaged (59 downed by fighters or flack, one ditched, 5 crashed on landing and 12 were so badly damaged they were considered a total loss and scrapped) along with 3 P-47 thunderbolt fighter escorts. There's just one problem with the claim of 186 kills. The luftwaffe kept pretty good records. The actual losses were a mere 38 fighters with a further 20 damaged but returned to service. The claim was inflated by 500%..
The Schweinfurt raid put a stop to daylight bombing of Germany for months. The 305th and 306th Bomb Groups lost 85% to 90% of their aircrews in a single mission and though the raid did severely damage the Schweinfurt ball bearing plant it had little or no effect to the German war effort and production was restored within 6 weeks.
Either way, a heavy bomber needed defensive armamants or it would have been a sitting duck to pick of for fighters at their leisure. At least with defensive armament an attacking fighter has to be more careful in its approach which makes it harder to shoot down the bomber or disrupt it.