While the defenses built into bombers were considered significant before the war, the reality of combat proved different.

Before the war, it was assumed that 'the bomber will always get through.' They were four-engined monsters, with multiple automatic weapons with wide arcs of fire. They could fly in formation to concentrate their fire. They could fly fast and fly high, making interception difficult. The theories of Douhet, Mitchell, and Trenchard (prewar theorists from Italy, the US, and Britain respectively) were backed up by tests that showed the devastation and accuracy of modern bombers.

Then the theories were tested in reality. Bombers were pretty easy to spot. You could use low-tech methods such as trained spotters on the ground equipped with telegraph or telephone lines. You could use high tech options such as radar to see the planes mere minutes after they took off. Then you could employ anti-aircraft artillery (AAA, or flak) to attack the bombers any time they were over your territory. These guns were more accurate and more effective than the interwar theorists assumed. Guns like the legendary German 88 were able to hit the bombers at basically any altitude, and a single hit was devastating if not a mortal blow.

Even worse for the bombers were the fighters. While the interwar development of fighters lagged a bit behind the bombers, by the beginning of WWII the fighters had caught up. They could go just as high as the bombers could, and they could go even faster. Also, they were armed with their own automatic weaponry. This was either machine guns, cannon, or a mix. The heavy machine guns could cripple a bomber engine in a single burst of only a fraction of a second. The same could be accomplished with a single cannon shell if it were well placed. Fighters merely had to aim their plane at their enemy to land a hit, while the gunners on the bomber had to adjust for the fighter's nimble movement as well as their own plane's movements. Bombing in daylight became unsustainable. The losses were so heavy that the force would be lost in a handful of missions, and so the switch to nighttime bombing was made by the British and Germans.

The Americans tried daylight bombing in 1943, and met the same results. Bombers were shot out of the sky faster than they could be replaced. They suspended daytime bombing until the new fighters like the P-51 (along with the P-47 and others) could be brought into the fray, and equipped them with drop tanks so they could make it all the way to the target and back. Now the bombers would still face deadly German fighters, but these fighters would be intercepted by the American escorts. Duty bound to try to defend their cities, the Luftwaffe challenged the Americans every time they appeared overhead. They lost a war of attrition fought in the skies of Germany, and eventually the Luftwaffe was crippled to the point where it could not contest the skies any longer. As such, there was only token aerial resistance during D-Day.

Planes like the legendary P-51 Mustang really were as advertised: critical to victory in the air. Whatever source you read was right on the money. Bombers could not survive hostile airspace without escorts.

Fighters that could stay with the bombers the entire way were important simply because the defensive armament of bombers was not sufficient enough as a defense against fighters.

It is incredibly hard to aim from a turret as opposed to forward mounted machine guns. Consider for instance that aircraft designers had toyed with, and used the idea of turreted guns as a primary armament for even fighters, and that this idea proved to be essentially worthless. As far back as WWI, turrets gave way to forward firing fixed machine guns on aircraft. But the idea didn't die there. The Bolton Paul Defiant was an ill fated aircraft in the lead up and early years of WWII. It was a fighter/interceptor that relied on a turret behind the pilot for it's primary armament.

That said, the defensive armament of bombers wasn't completely useless. It still made it dangerous for enemy aircraft to attack, and they had to adapt tactics and even weapons for handling them. The Germans for instance found that head on attacks proved the least lethal towards attacking fighters, yet at the same time head on attacks are very difficult simply due to the closing speeds. They also started outfitting fighters with longer range weapons such as heavier cannon and even rocket mortars as a means to avoid the defensive armament of the bombers.

One problem with evaluating the defensive armament is that bomber crews very often over-reported their own effectiveness. It wasn't necessarily out of arrogance or anything like that, but when you have multiple people firing upon the same target, you may very well have many people claiming victories. So while the Germans may lose one aircraft in a sortie to defensive armament, you might have 3 different gunners from different bombers claiming the kill, and as such the numbers get inflated.

edit: Something else I can't believe I forgot to mention that played a major role. I've bolded/embiggened the next sentence because it really is huge in terms of the air war in WWII on the Western Front (and beyond of course). The fact that the fighters could escort the bombers all the way also meant the bombers were not always on the defensive. The final nail for the Luftwaffe came when Doolittle "released the fighters". By this, it is meant that he told the fighters to go out and actively seek the Luftwaffe wherever they could be found, in the sky or on the ground. No longer were the Allies simply waiting for the Germans to come up to engage them, but rather they were actively seeking them out. Fighter pilots would often escort the bombers and stay relatively close so as to defend them, but they could also range out and find the Luftwaffe before they had a chance to really engage the bombers. You also often had fighters strafing the enemy on the ground during the return leg of their mission. Fighters could also range out well ahead of the bombers performing fighter sweeps, which would clear the skies ahead of the bombers. The Allies also started using tactics to lure the Luftwaffe into the sky as well. They might mimic the flights of bombers in order to lure the Luftwaffe into the sky, but instead the Luftwaffe would find giant waves of fighters waiting for them instead of those big fat juicy bomber targets.

edit2: Sorry, I should have gone more in depth why turrets aren't effective. Okay, so basically it's just much easier to aim with forward firing fixed guns as opposed to a turret (for now on I will reference forward firing guns simply as "fighters" for brevity's sake). So, the fighters' guns go straight forward relative to the aircraft. Turret guns however, may have to deal with the wind and movement of the aircraft. If you shoot 90 degrees straight to the left, the bullets are going to behave in a much less predictable way. This makes it much harder to "lead the target". Leading the target is where you fire where the target will be when the bullet gets there. Since it takes time for the bullets to travel, you can't just point the barrel of a gun straight at a moving target and expect to hit it. Instead, your bullets will fall behind the target unless you compensate for this.

Even with tracers, it can still be hard to see exactly where your bullets are going when firing from a turret, and it can be hard to judge exactly where in space they are even though you can follow the tracer. With a fighter, you can much more predictably guess where the bullets will end up, since your aircraft and the bullets are mostly traveling "together" so to speak. In a turret, say you're firing straight up into the air, while the aircraft is traveling forward. Your bullets are going to appear to arc towards the rear of the aircraft, because they do not perfectly maintain the momentum of the aircraft at all, and will instead be hit by a wall of wind that is going to drastically affect their trajectory as well.

Further compounding the issue is that many defensive armaments had limited fields of fire, and the gunners often had very limited vision. Dorsal and ventral (top and bottom) turrets may offer nearly 360 degrees of lateral and 90 (which equates to 180 when you factor in the lateral movement) degrees of movement, but their vision is still limited. Even powered turrets also struggled to keep up with tracking fast moving targets. Waist gunners on aircraft like the B-17 had it even worse, in that they had to manually aim their guns through a very limited field of fire with a very limited field of view. Radio and even just verbal shouting amongst crew members helped alleviate the narrow vision aspect to a degree, in that one gunner could call out to another to expect a target to appear from a certain direction, but this only helped so much.

Finally, turrets at best generally only had a pair of .50 caliber machine guns. Often, they might only have a single .50 cal (smaller aircraft might have a pair of .30/.303/7.92mm or even just a single gun of those types). While the combat box formations used by bombers could bring more guns to bear on the enemy, in reality only a handful of guns at any given time might be attempting to track a target. The odds of hitting said enemy due to all the aforementioned reasons were very low. So even if a turret managed to place rounds on target, at best you're probably looking at a pair of .50 cals making actual direct hits.

The answers already given are very good, and accurate, but I would add something else to them to flesh this out. A machine gun mounted in a turret or opening in the side of a bomber provides what is called a cone of fire. This cone is the area you can strike with bullets from that position by raising, lowering, or sweeping side to side the maximum amount. Everything within the circle you make of your weapons max traverse in all 4 directions is in the cone of fire. It is called a cone because it expands with distance away from the weapon. The location of the defensive machine guns on a bomber determines the total coverage of these cones of fire, and determines where the coverage is maximized by bringing multiple weapons onto an attacker coming in from certain angles. It also determines where the weak spots are.

The B-17 had a tail gunner, 2 waist gunners located in the fuselage between the wings and tail shooting out each side of the aircraft, a ball gunner on the bottom of the aircraft, and a top gun aimed by the engineer. Later models had a front gun slung low in the nose (initially they didn't put a turret there to keep a clear line of sight for the bombardier). Here is a basic diagram: http://www.intolifesschool.com/b-17.html

There were two primary "soft spots" in a B-17's defenses. One is attacking head on. From that angle the tail waist and ball turret gunners are all out of the fight. On early models of the B-17 only one gun could be brought to bear, the top turret and that at a bad angle. German pilots generally attacked from slightly above and in front of the plane to give them a better sighting angle to shoot, this is what the movies call the "12 o'clock high" attack. The other best place to hit a B-17 is diving straight down on it from directly above. Like in the frontal attack this means only 1 turret can engage defensively, and during daylight the gunner would be staring straight up into a bright sky which is difficult to do.

Even by adjusting tactically to the B-17 and finding the soft spots to hit it this was not easy to do. B-17's flew in boxed formations, and each formation had hundreds of gunners fighting off very sturdy, rugged bombers. It took a lot of courage to fly into that particular hornets nest, and the Germans lost a lot of pilots doing so. They killed a lot of American Bombers in the process too. The American 8th Air Force took heavier casualties in daylight bombing raids than the troops storming the beach at Normandy did.

Ultimately, as tough as the bombers were built (B-17's were notoriously hard to drop) and as heavily defended as they were if you let German Pilots line up carefully and attack at the right angle a percentage of those attacks would be successful. The best defense in this case was to impede the Germans from lining up these carefully aimed attacks in the first place, make them fight for the approach and come in at bad angles where they were less likely to succeed. This takes fighter cover. Before fighters could escort bombers all the way casualties were horrendous. Once they could and German pilots had to work for every firing pass on the formations casualties dropped tremendously.