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Bigger WWII bombers like e.g. the B-17 Flying Fortress were equipped with lots of machine guns for self-defense against single-seat fighter aircraft.

Wikipedia says:

Defensive armament increased from four 0.50 in (12.7 mm) machine guns and one 0.30 in (7.62 mm) nose machine gun in the B-17C, to thirteen 0.50 in (12.7 mm) machine guns in the B-17G. But because the bombers could not maneuver when attacked by fighters, and needed to be flown straight and level during their final bomb run, individual aircraft struggled to fend off a direct attack.

The last sentence and the fact that fighter escort was developed and also personal impression from live footage etc. indicates that the self-defense armament wasn't very effective. Is this true?

I can understand that bombers cannot do evasive maneuvers, but so does the attacking fighter plane when it is homing in on the bomber and aiming, isnt it? In that moment the bomber actually should have the advantage of being able to point the (several) mobile guns, while the fighter must use the entire airframe to aim its guns and thus has a very predictable flight path.

And in case it really wasn't effective, why did they carry it then? Must have added a lot of additional weight.

Are there any figures to illustrate the effectiveness of bomber self defense?

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    $\begingroup$ There's a big difference between "struggled" and "totally failed." Moreover, live footage would tend to be biased. $\endgroup$ – David K Mar 20 '18 at 19:34
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The only figures I could find were the 60 bombers out of 300 lost by the USAAF in the Schweinfurt raid, most of those losses from German fighters. That convinced the generals that the onboard gunners weren't all that effective, and bombers couldn't defend themselves from a determined fighter attack. Major air attacks on Germany were postponed until the long range P51 could be deployed to protect the bombers better.

Gunnery from a bomber is far more difficult than from a fighter, that fires straight ahead. In a bomber, the gunner might be firing to the side, which means they have to factor in the bomber's speed that will propel the bullets sideways, and actually aim slightly behind the approaching fighter. The closer to the nose or tail, the less one aimed behind... bomber gunnery is very complex. This wartime training film illustrates the problems of firing on fighters approaching from the side. Try to calculate those ever changing angles when you're being shot at...

The same difficulties were experienced by ships being attacked by aircraft... very difficult to shoot down a small, fast moving aircraft approaching a ship. The Bismarck couldn't stop slow moving Swordfish biplanes from knocking it's rudders out. In the last major naval action of the war, eight Japanese warships (including the Yamato) were attacked by around 400 aircraft from US carriers. Seven of the eight ships were sunk, while only 14 aircraft were lost in the action, despite those ships having had quite a few extra AA guns added.

So, why arm the bombers? That was a question asked by Curtis LeMay in 1945. He devised a plan to take most of the guns out of B29's and operate them at night, because the Japanese hadn't demonstrated much in the way of night interception. He reduced the attack altitude to 12k feet from 30k feet, because that put the planes above small arms, but below the effective range of large anti-aircraft cannon. And, he used the weight saved from armament removal to carry extra incendiary bombs, to ignite the largely wood buildings of major Japanese cities.

The result was the fire raids, the most destructive bombing campaign of the war, even more destructive than the atomic bombs.

Guns on bombers have been dwindling since then, with the B52 having only one tail gun, while the B1 and B2 have no guns. Still, during Linebacker II, two MIG21's were shot down by B52 tail gunners, so the guns weren't completely useless.

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  • $\begingroup$ Gunners do not "factor in the bombers speed". It is only relative Line of sight rate that affects the gunnery solution aim point. There is an effect referred to as :"Drag Shift" which requires that you aim in front of a target aircraft flying co-speed next to you, but this is not due to his airspeed, it is due to the deceleration of the bullet over it's time of flight. If you were firing a rocket, which accelerates over its time of flight, you would actually need to aim behind the target. $\endgroup$ – Charles Bretana Mar 22 '18 at 14:22
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    $\begingroup$ @CharlesBretana, the drag shift is not due to deceleration itself, but due to the relative wind accelerating it laterally in your frame of reference. This lateral acceleration applies equally to rockets, so even rockets must be aimed in front of a target to compensate for drag shift. $\endgroup$ – Jan Hudec Mar 24 '18 at 21:37
  • $\begingroup$ Apologies, @Jan Hudec, but you are wrong. If a projectile had just enough thrust to maintain velocity, you could aim directly at the target. ALL drag on the projectile moves it in the direction opposite to its relative wind. BY DEFINITION, this drag (to whatever degree it exists), can only change projectile true air speed velocity parallel to its flight path. It CANNOT change velocity across its flight path. This is why, when bombing in a crosswind, aircraft must fly directly over the target for low drag bombs, upwind when dropping high-drag weapons, and downwind when firing rockets. $\endgroup$ – Charles Bretana Mar 25 '18 at 14:18
  • $\begingroup$ The "Training Film" mentioned in the answer is, in fact, also wrong. It is not the bombers forward velocity that causes the need top aim "behind" the attacking fighter. If you watch the film carefully, you will notice that the fighter is drifting backwards, towards the tail of the bomber in its attack profile. It is this rearward relative Line of sight rate that creates the need to aim towards the rear. The fighter is drifting rearwards because its current straight line flight path would carry it behind the bomber. The fighter is doing this so he can be turning forward when he fires. $\endgroup$ – Charles Bretana Mar 25 '18 at 14:43
  • $\begingroup$ If taught, and understood, properly, it is really not as hard as presented above, or, in the WWII training film. All you need to do is observe the relative line of sight (LOS) rate, or the apparent motion, of the target against the infinite backdrop. You can easily see that, even in the WWII training film. Then all you need to do is aim the gun in "front" of that motion, by an amount proportional to speed of that LOS rate, and the time of flight (distance to TGT), and a little bit above for gravity drop. $\endgroup$ – Charles Bretana Mar 25 '18 at 14:48
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While both the fighter and the bomber fly straight, the bomber for the bombing run and the fighter to keep its guns on the bomber, the bomber is far easier to spot for the fighter pilot, than it is for the gunners in the bomber to spot the fighter.

However, as the gunners of the bomber shoot back at the fighter, the fighter can't afford to take his time to get a good aim at the bomber. Thus, defensive fire from the bombers greatly reduced the fighters' time to take a shot at the bomber.

I have no figures on effectiveness, however, the Luftwaffe developed tactics specifically to avoid staying in the field-of-fire of the defensive guns. That should give some indication; if it is worth it to modify aircraft to avoid the defensive guns of bombers, then the defensive guns were considered a threat to the fighters.

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    $\begingroup$ Tactics adopted by the Luftwaffe to deal with USAAF bomber formations included head on attacks, which minimised exposure to defensive gunfire, and the use of rockets to engage the bomber formations from outside of the effective range of their .50 calibre machine guns. $\endgroup$ – J. Southworth Mar 22 '18 at 14:02
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    $\begingroup$ The fighters also had more firepower. The bomber generally only had the .50 caliber (12.7mm) machine guns with solid bullets, while fighters mostly switched to 20mm or 30mm cannons with explosive ammunition, which did a lot more damage on hit. $\endgroup$ – Jan Hudec Mar 24 '18 at 22:03
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    $\begingroup$ @J.Southworth: This resulted in a bomber pilot making ace because he happened to have a nose gun while most didn't. $\endgroup$ – Joshua Dec 13 '18 at 16:11
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    $\begingroup$ It seems improbable to me that a bomber pilot would have been able to destroy five fighters with one machine gun. I don't buy it. He may have claimed five, but it's been shown that claims by bomber crews for enemy aircraft destroyed were often exaggerated. $\endgroup$ – J. Southworth Feb 12 at 20:58
  • $\begingroup$ I read about one instance in which a Ju88 night fighter destroyed two P51 Mustangs during a day mission. The Ju88 was designed originally as a bomber, but developed into fighter versions as well. $\endgroup$ – J. Southworth Feb 12 at 21:08
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The question of the effectiveness of bomber armament during the Second World War has to be assessed in the context of the bombing tactics used rather than in isolation. The tactics used by the USAAF units operating the B17 in Europe were based on the principle that bombers flying in formation could combine their firepower to effectively defend themselves against any attack. This proved incorrect, and heavy losses occurred, necessitating the provision of fighter escort. USAAF bomber losses on daylight missions were however lower than those suffered by RAF Bomber Command in night missions, in which the individual aircraft flew alone and if intercepted, had little chance against a heavily armed twin engined night fighter. Night bombing tactics were adopted by the RAF as a result of early combat experience in which their medium and heavy bomber aircraft proved excessively vulnerable in daylight missions, mainly because of their inadequate performance and defensive armament. At the beginning of the war, the official but nevertheless naive RAF doctrine was that bombers could effectively defend themselves against fighter attack if equipped with power operated gun turrets, even if these were only armed with rifle calibre machine guns.

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