On August 17,1942, United States B-17s carried out a bombing raid on the railway yards at Rouen in France. The real offensive, however, started on January 27,1943, when B-17s of the USAAF made their first attacks on Germany. Initially, casualties were very high because they attacked during daylight hours to achieve greater accuracy, and because proper formation flying (to enable a group of airplanes to defend each other with crossfire) had not yet been formulated. Also the B-17F lacked adequate defense against a head-on attack. By September 1943, the Flying Fortress showed its final shape during firepower tests on the XB-40, a modified B-17F with the advantage of a "chin" turret. The success of the chin turret, led to the delivery of the B-17G (the major production version), which was the first production variant to have a chin turret installed, under the nose. The Bendix turret held two .50-cal. guns, which increased the armament to thirteen guns.9 In all, there were 8,680 B-17Gs built by Boeing, Vega, and Douglas to make this the largest production variation. Produced in greater numbers than any other single model, more B-17Gs were lost, than any other model. B-17G specifications included a span of 103 feet 9 inches (31.6 m), length of 74 feet 4 inches (22.6 m), and a height of 19 feet 1 inch (5.8 m). The four supercharged Wright R-1820-97 Cyclones delivered 1,200 hp (895 kW) and gave a top speed of 287 mph (462 km/h), cruising at 182 mph (293 km/h). Service ceiling was 35,800 ft. (10,850 m), with a max. range of 3,400 miles (5,471 km). Empty and gross weights were 36,135 lb (16,390 kg)and 65,500 lb (29,710 kg). Maximum fuel load was 3,630 gallons (13,741 liters).
On 19 July 1943, US B-17s and B-24 Liberators carried out the first bombing raid on Rome. US bombing in Europe reached its high point in February 1945 with a 1,000-bomber raid on Berlin, escorted by 400 fighters, and the Dresden raid (alongside RAF Lancasters) which, caused a massive fire storm to sweep the city. Meanwhile, B-17s were also helping to win the war against Japan, although by mid-1943 the larger Boeing B-29 had begun to take over the major strategic bombing missions in the Pacific theater. Following the first Model 299, the Air Corps purchased 12,725 B-17 type aircraft, of which a few served with Royal Air Force Coastal Command and the United States Navy for patrol, air-sea rescue, antisubmarine and other duties. Cargo conversions of the B-17 were known as the XC-108.