Boeing B-17 Flying Fortress

At the start of the Second World War, the United States Army Air Force (USAAF) saw, as the mainstay of strategic land bombing, and indeed a significant threat to shipping, the Boeing-designed B-17 Flying Fortress. Although originally designed in 1934, it was ahead of its time and proved a effective heavy bomber, within constraints that applied to all bombers tested in peacetime conditions, and in the absence of serious air defense. The Boeing corporation built 6,981 B-17s in various models, and another 5,745 were built under a nationwide collaborative effort by Douglas and Lockheed (Vega). They were mainly deployed in Europe, flying with the Eighth Air Force in England and the Fifteenth Air Forcein the Mediterranean.

(CC) Photo: Andrew Pescod
The "Sally B" is a B-17G best known for its role in the movie "Memphis Belle."

Technology

The B-17 was designed in the mid 1930s to accomplish the Air Corps mission of long-range strategic bombing. The problem was that War Department and the Navy Department did not recognize that mission. Given the isolationist mood that precluded building offensive weapons, the B-17 had to be billed as a "defensive" weapon to defend the coastline. Perhaps this was the explanation of why it was presented as an anti-shipping weapon, since the Norden bombsight had no capability against a maneuvering target.

It was a highly adaptable aircraft, and was continually being improved based on engineering creativity and operational experience. continuous improvements added power gun turrets, bullet-proof glass, self-sealing fuel tanks, high-altitude oxygen systems, enlarged wing and tail surfaces, better radios, and ground- scanning radar. The power plant was upgraded with turbo- superchargers and 1,200 horsepower Wright Cyclone engines operating on 100 octane fuel (and later, 110/145 grade). The service ceiling rose to 35,000 feet, the range to 3,300 miles with a 4,000 pound payload.

The aircraft and bombsight worked well enough in leisurely practice runs in sunny California at 10,000 feet with no flak or enemy planes. Over German airspace, in bad weather at 20,000 feet with shells exploding all around and enemy fighters a constant threat, the B-17 had at most 30 seconds over the target. "The flak is murder," the pilots said. "If you fly straight and level through it for more than ten seconds, you're a dead duck." Furthermore, it was hard to find the target in the first place. Navigation errors often put streams of 500 bombers many miles the wrong direction. At high altitude, with the usual cloud cover, it was nearly impossible to identify urban landmarks visually. On clear nights camouflage and dummy cities confused the navigators.

Initial operations

General Ira Eaker's Eighth Air Force first launched its heavy bombers against Hitler in August, 1942. The main priority until 1944 was the destruction of Luftwaffe planes in the air and on the ground. One year later, after 83 major missions aimed at France, Holland, and the German cities closest to the English Channel, Eaker sent 376 B-17s against the vital ball bearing factories at Schweinfurt and the Messerschmitt factory at Regensburg. Both small cities were located deep in Germany, far out of range of the P-47 and Spitfire fighters that normally escorted the bombers. German fighters and flak downed 60 bombers--a half dozen more raids like that and the 8th Air Force would cease to exist. After 30 more peripheral raids the Eighth tried Schweinfurt again in October, and again 60 Flying Fortresses (out of 320) were shot down. Accuracy was good despite the fierce resistance, and damage was heavy. The Germans took several months to rebuild (and to disperse critical plants so one raid would not prove fatal.)

In daylight, large formations of several hundred B-17s were easily spotted. For self-defense each Flying Fortress had 13 50- calibre machine guns, and flew in loose formations of 6 planes, each covering the others. In 1942-43 the Luftwaffe proved the Fortresses were vulnerable. Unexpectedly heavy German flak defenses disrupted formations, and damaged on average one-fourth of the bombers in each mission. Berlin was surrounded by an outer searchlight belt 60 miles in diameter, and a flak area 40 miles across. The searchlights helped the guns locate their targets and also blinded the navigators. Three massive 120-foot flak towers resembling medieval castles protected central Berlin with 8 128mm high velocity guns each. They fired a salvo every 90 seconds that created a killing window 260 yards across in the path of the bombers.

Hit by flak, some bombers crashed, while others fell out of formation; the stragglers were easy prey for fighters. The Luftwaffe moved fighters from the Eastern Front to the West. Improved German radar, new airfields, and centralized ground control allowed groups of fighters to be quickly vectored into the predicted flight path of the bombers. The fighters discovered the best way to attack was head-on ("Twelve O'Clock High!") because the very fast closing speed gave the B-17 gunners only a split second to aim, while the fighter pilot could aim his machine guns by pointing his entire aircraft at the bomber. The B-17 loss rate climbed from 3.5% per sortie in 1942 to 5% in early 1943. The B-17 was a robust plane able to withstand heavy punishment, but when 5% were lost in a single mission, the life expectancy per bomber was a mere 13 missions. The Luftwaffe was winning this war of attrition. New defensive techniques included forward-firing chin turrets, tighter formations of 18 planes, and deceptive diversionary attacks; they were not enough.

Delivering the bombs

Before aiming the bombs, the aircraft had to find the target. Weather could prevent the bombers from taking off from Great Britain, or, once airborne, getting into formation with other bombers and, if available, escort fighters (e.g., P-51)

Navigators found it very difficult to find the target in the first place. Navigation errors often put streams of 500 bombers many miles the wrong direction. At high altitude, with the usual cloud cover, it was nearly impossible to identify urban landmarks visually. On clear nights camouflage and dummy cities confused the navigators.

Modern readers should understand that the Norden bombsight offered nothing close to the accuracy of modern precision-guided munitions; do not think of it as the deadly video games of the Gulf War and more recent conflicts where a bomb might go through a designated window or hit the center of the top of a tank turret. Under ideal conditions, the Norden could deliver 50 percent of its bombs within 1000 feet of the desired aiming point, once the bomber was over the target

The Norden bombsight allowed daylight bombing of specific targets of the size of substantial factories. While its users swore a special oath of secrecy:

It worked well enough in practice runs in sunny California at 10,000 feet with no flak or enemy planes. Over German airspace, in bad weather at 20,000 feet with shells exploding all around and enemy fighters a constant threat, the conditions for effective Norden user were rare. To work as intended, the Norden required the aircraft to fly straight and level for 30 seconds. "The flak is murder," the pilots said. "If you fly straight and level through it for more than ten seconds, you're a dead duck."

It helped locate targets, but beginners' luck in its early trials gave planners a much exaggerated estimate of the accuracy the Flying Fortress could achieve. In late 1943, one bomber in 25 hit within one mile of the aiming point, and only one in 5 even got within five miles. If the aiming point was a factory or railroad yard, fewer than 10% of the bombs that did land there would do any real damage. Bombs that missed and landed in residential areas were just like the RAF's; they would destroy apartments, but the residents were usually safe in underground shelters.

German defenses

The lessons were a profound blow to American strategic bombing doctrines-- the British warnings about the devastation fighters could wreak on unescorted daytime bombers had proven correct. Luftwaffe clearly had air superiority over the Nazi heartland, unescorted bombers would suffer unacceptably high losses, and even severe damage could be quickly repaired. In daylight, large formations of several hundred B-17s were easily spotted. For self-defense each Flying Fortress had 13 50-calibre machine guns, and flew in loose formations of 6 planes, each covering the others. In 1942-43 the Luftwaffe proved the Fortresses were vulnerable. Unexpectedly heavy German flak defenses disrupted formations, and damaged on average one-fourth of the bombers in each mission. Berlin was surrounded by an outer searchlight belt 60 miles in diameter, and a flak area 40 miles across. The searchlights helped the guns locate their targets and also blinded the navigators. Three massive 120-foot flak towers resembling medieval castles protected central Berlin with 8 128mm high velocity guns each. They fired a salvo every 90 seconds that created a killing window 260 yards across in the path of the bombers. Hit by flak, some bombers crashed, while others fell out of formation; the stragglers were easy prey for large fighters with heavy cannons and rockets.

The Luftwaffe moved its best pilots and fighters from the Eastern Front to defense of the homeland. Improved German radar, new airfields, and centralized ground control, an integrated air defense system called the Kammhuber Line allowed groups of fighters to be quickly vectored into the predicted flight path of the Allied bombers. Luftwaffe ace Egon Mayer demonstrated the best way to attack was head-on ("Twelve O'Clock High!") because the very fast closing speed gave the B-17 gunners only a split second to aim, while the fighter pilot could aim his machine guns by pointing his entire aircraft at the bomber. The B-17 loss rate climbed from 3.5% per sortie in 1942 to 5% in early 1943.

The B-17 was a robust plane able to withstand heavy punishment, but when 5% were lost in a single mission, the life expectancy per bomber was a mere 13 missions. The Luftwaffe was winning this war of attrition. New defensive techniques included forward-firing chin turrets, tighter formations of 18 planes, and deceptive diversionary attacks; they were not enough. One abortive attempt was the YB-40, a B-17 that carried no bombs, but extra armor and guns as a "flying battleship". Unfortunately, after the real bombers had dropped their payload, the YB-40 still carried the weight of guns and armor and could not keep up with them.

Fighter escorts

In late 1943 the AAF suddenly realized the need to revise its basic doctrine: strategic bombing against a technologically sophisticated enemy like Germany was impossible without air supremacy. General Arnold replaced Ira Eaker with Carl Spaatz and Jimmy Doolittle, who fully appreciated the new reality. They provided fighter escorts all the way into Germany and back, and cleverly used B-17s as bait for Luftwaffe planes, which the escorts then shot down. Doolittle's slogan was "The First Duty of 8th AF Fighters is to Destroy German Fighters." In one "Big Week" in February, 1944, American bombers protected by hundreds of fighters, flew 3,800 sorties dropping 10,000 tons of high explosives on the main German aircraft and ball-bearing factories. The US suffered 2,600 casualties, with a loss of 137 bombers and 21 fighters. Ball bearing production was unaffected, as Nazi munitions boss Albert Speer repaired the damage in a few weeks; he even managed to double aircraft production. Sensing the danger, Speer began dispersing production into numerous small, hidden factories.

Paradoxically, the Luftwaffe would have to come out and attack or see its planes destroyed at the factory. Before getting at the bombers the Germans had to confront the more numerous, better armed and faster American fighters. The heavily armed BF-110 could kill a bomber, but it slowness made it easy prey for the speedy P-47 Thunderbolts and P-51 Mustangs armed with numerous fast-firing machine guns. The big, slow twin-engine Ju-88 was dangerous because it could stand further off and fire its rockets into the tight B-17 formations; but it too was hunted down. Germany's severe shortage of aviation fuel had sharply curtailed the training of new pilots, and most of the instructors had been sent into battle. Rookie pilots were rushed into combat after only 160 flying hours in training compared to 400 hours for the AAF, 360 for the RAF and 120 for the Japanese. They never had a chance against more numerous, better trained Americans flying superior planes.

The Germans began losing one thousand planes a month on the western front (and another 400 on the Eastern Front). Realizing that the best way to defeat the Luftwaffe was not to stick close to the bombers but to aggressively seek out the enemy, Doolittle told his Mustangs to "go hunting for Jerries. Flush them out in the air and beat them up on the ground on the way home." On one occasion German air controllers identified a large force of approaching B-17s, and sent all the Luftwaffe's 750 fighters to attack. Error. The bogeys were all Mustangs, which shot down 98 interceptors while losing 11. The actual B-17s were elsewhere, and completed their mission without a loss. In February, 1944, the Luftwaffe lost 33% of its frontline fighters and 18% of its pilots; the next month it lost 56% of its fighters and 22% of the pilots. April was just as bad, 43% and 20%, and May was worst of all, at 50% and 25%. German factories continued to produce many new planes, and inexperienced pilots did report for duty; but their life expectancy was down to a couple of combat sorties. Increasingly the Luftwaffe went into hiding; with losses down to 1% per mission, the American bombers now got through and Luftwaffe tactical air power had vanished.

In early 1943 the strategic bombers were directed against U-boat pens, which were easy to reach and which represented a major strategic threat to Allied logistics. However, the pens were very solidly built--it took 7,000 flying hours to destroy one sub there, about the same effort that it took to destroy one-third of Cologne. In "Operation Crossbow" about a fourth of the bombers were reassigned to attacks on Germany's V-1 and V-2 bases in 1944. The raids were ineffective. The antisubmarine and Crossbow campaigns thus were a "victory" for the Germans because they wasted Allied airpower.

From April through August, 1944, both the AAF's and the RAF's strategic bombers were placed under Dwight D. Eisenhower's direction, where they were used tactically to support the invasion. Airmen protested vigorously against this subordination of the air war to the land campaign, but Eisenhower forced the issue and used the bombers to simultaneously strangle Germany's supply system, burn out its oil refineries, and destroy its warplanes. Mission accomplished, Ike returned the bombers in September. The Flying Fortresses continued their mission of demolishing German's industrial plant and rail system until April 1945.

Results

Germany was burned out and lost the war in large part because of strategic bombing. Targeting became somewhat more accurate in 1944, but the real solution to inaccurate bombs was more of them. The AAF dropped 8 million bombs (1.6 million tons) against Germany using the B-17 and B-24. Besides knocking out the Luftwaffe, the second most striking achievement of the strategic bombing campaign was the destruction of the German oil supply. Oil was essential for U-boats and tanks, while very high quality aviation gasoline was essential for piston planes. Germany had few wells, and depended on imports from Russia (before 1941) and Nazi ally Romania, and on synthetic oil plants that used chemical processes to turn coal into oil. Heedless of the risk of Allied bombing, the Germans had carelessly concentrated 80% of synthetic oil production in just 20 plants. These became a top priority for the AAF and RAF in 1944, and were targets for 210,000 tons of bombs. The oil plants were very hard to hit, but also hard to repair. The strategic bombings by the AAF and RAF dried up the oil supply in the summer of 1944. An extreme oil emergency followed, which grew worse month by month.

The third notable achievement of the bombing campaign was the degradation of the German transportation system--its railroads and canals (there was little truck traffic.) In the two months before and after D-Day the American Liberators (B-24), B-17 Flying Fortresses and British Lancasters hammered away at the French railroad system, complemented by Resistance fighters sabotaged some 350 locomotives and 15,000 freight cars every month. Critical bridges and tunnels were cut. Berlin responded by sending in 60,000 German railway workers, but even they took two or three days to reopen a line after heavy raids on switching yards. The system deteriorated quickly, and it proved incapable of carrying reinforcements and supplies to oppose the Normandy invasion. To that extent the assignment of strategic bombers to the tactical job of interdiction was successful.

When Bomber Command hit German cities, it inevitably hit some railroad yards. The AAF made railroad yards a high priority, and gave considerable attention as well to bridges, moving trains, ferries, and other choke points. The "transportation policy" of targeting the railroad system came in for intense debate among Allied strategists. It was argued that enemy had the densest and best operated railway system in the world, and one with a great deal of slack. The Nazis systematically looted rolling stock from conquered nations, so they always had plenty of locomotives and freight cars. Furthermore, most traffic was "civilian," and urgent troop train traffic would always get through. The critics exaggerated the resilience of the German system. As wave after wave of bombers blasted away, repairs took longer and longer. Delays became longer and more frustrating.

Troop trains usually got through, but the "civilian" traffic that did not get through comprised food, uniforms, medical equipment, horses, fodder, tanks, fuel, howitzers, flak shells and machine guns for the front lines, and coal, steel, spare parts, subassemblies, and critical components for munitions factories. By January, 1945, the transportation system was cracking in dozens of places, and front-line units had more luck trying to capture Allied weapons than waiting for fresh supplies of their own.