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The centerpiece of America's strategic bombing was the [[B-17 Flying Fortress]]. Although originally When designed in 1934, it was ahead of its time (in 1934 - not in 1939) and proved a reasonably effective heavy bomber. A policy of 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 46,000 pound (normal) bomb payload. The AAF considered B-17 the perfect embodiment of its strategic bombing doctrine because of its long- range, its ability to defend itself, and its highly accurate Norden bombsight. The Norden allowed daylight precision bombing of specific targets like factories. It 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.<ref>Cloud cover over Germany averaged 50-80%. In winter, a severe storm occurred every three days; early morning fog covered airfields in England every third morning. In 1944, the 8th Air Force was able to fly from Britain on only 200 days.</ref> H2X, an American adaptation of the British radar system H2S, provided a crude mapping of the ground through cloud cover. 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. <ref> Craven & Cate 3:20] LeMay told of flak, 314 </ref> This was exacerbated by the B-17's very small smaller bombload. Only 26,000 pounds could be carried without restricting range severely. In contract , the RAF's [[Avro Lancaster]] could carry 14,000 pounds to maximum range and by removing the bomb bay doors could increase this to a maximum of 22,000 pounds; one Lancaster had the offensive power of at least seven B-17s.