In May 1934, the US Army published the conditions with which it should meet the modern multi-engine bomber it needed. It was required that it could take up to 907 kg of bombs on a route from 1640 to 3540 km, while maintaining a speed of 322 to 402 km / h. While, according to the US Army, the term multi-engine meant that there should be more than one engines, Boeing invited to present its concept adopted the use of four engines for the Boeing 299 (later designated B-17). Work on the machine began in mid-June 1934. On July 28, 1935, the Boeing 299 took off for its first flight. It took place on a 3,380 km route with an average speed of 406 km / h. The news of the positive test results made Boeing optimistic. The more bitterly news was received that on October 30, 1935, the prototype had crashed during take-off. The study of the causes of the crash revealed that the take-off took place with the control system blocked (normal procedure when the aircraft was stationary). Nevertheless, taking into account the positive results of the tests carried out, the army decided to place an order for 13 YB-17s and one for static tests. The prototype that crashed was powered by four Pratt-Whitney R-1680-E Hornet engines with a power of 750 HP each. Its wing at the base was of such a high profile that it reached half the diameter of the circular fuselage. There were large-span flaps on the trailing edge of the wings, which reduced the speed during take-off and landing. The undercarriage with the rear wheel was electrically retractable and extended. The armament consisted of five machine guns, and the mass of the bomb load carried in the hull hatch reached 2177 kg. The first flight of the YB-17 took place on December 2, 1936. The aircraft differed from the prototype by the use of Wright GR-1820-39 Cyclon radial engines with a power of 930KM, preparation for flight with a crew of 9 and a number of minor improvements. 12 aircraft were delivered between January and August 1937. They are part of the equipment of 2 Bombardment Group in Langley Field, Virginia. The 13th copy was delivered to Wright Field as scheduled for a series of tests to be completed. However, after one of the Y-B17s left the flight without damage in strong turbulence, it was decided to bring the thirteenth copy to the operational standard. The aircraft, designated U1B-17A, was powered by 1000HP GR-1820-51 radial engines, equipped with exhaust gas powered Moss / General Electric turbochargers. The first flight of this machine took place on April 29, 1938. The operating experience has proved the advantages of such a power unit over naturally aspirated engines, and from then on, only turbocharged engines were used for all subsequent Flying Fortress machines. After the order for the YB-17, a contract was signed for the construction of 39 B-17Bs – a variant similar to the first version, but with 1200HP R-1820-65 turbocharged engines and an increased number of machine guns – up to 7 pieces. The first aircraft took off on June 27, 1939, and the delivery of all aircraft was completed in March 1940. The B-17C was the first version of this aircraft delivered to the RAF in Great Britain. The first 20 units shipped at the beginning of 1941 were designated Fortress I. They were placed in the equipment of the 90 squadron, which included them for the first time operationally used on July 8, 1941 during the bombing of Wilhelmshaven. During the next two months, however, the B-17s, taking 26 actions, turned out to be ineffective, with the Americans believing that it was due to the improper use of the new bombers. Although the Bf-109 Messerschmitts had problems intercepting the B 17 during the flight at the maximum altitude of 9,750m, it turned out that during the daytime operations over Germany, the flight altitude alone was not sufficient defense, and thus it was necessary to mount a stronger armament. Pending changes to equipment or the development of better ways to use these machines, they have been phased out from flying over Europe. With the end of 1941, the United States was engaged in hostilities. Initially in the Pacific, and later after the violent Japanese expansion was halted. In Europe, where the Allies concluded that they must concentrate their efforts to end the war on the old continent. As a result, a large number of B-17s that were to be found in the Far East were redirected to Great Britain and re-supplied by USAAF’s 8 Air Force. In 1940, Boeing received an order for 42 B-17Ds. They differed from the B-17C only slightly – they were equipped with self-sealing fuel tanks and additional crew cabin armor. The machines were delivered in 1941. The B-17E, F and G airplanes had a redesigned and enlarged tail, and they differed from their predecessors mainly by the powerful tail fin. The B-17 E and F were the first of these bombers to be included in the aforementioned 8 Air Force in Europe. They were at that time the most modern B-17 machines, yet in two main operations on August 17 and October 14, 1943 as many as 120 planes were knocked down against German strategic targets. It turned out that the Fortress cannot provide sufficient cover for themselves, no matter how tightly in the formations they fly. It was a painful truth, but these planes were indeed vulnerable in daytime operations, unless they were accompanied by long-range escorts. Many of the B-17s were shot down during a frontal attack, and therefore the last production version was redesigned so that the aircraft could withstand similar attacks. As a result, the B-17 G had a turret in the lower bow section to accommodate two 12.7 mm machine guns. This increased the number of defensive weapons to 13 machine guns. Not only was the majority of the B-17s used in Europe and the Middle East, but the aircraft took part in combat wherever US units fought. Over the Pacific, the B-17s have played an invaluable role in naval patrol flights, reconnaissance and classic bomber or close support missions. Of the total of nearly 13,000 different B-17s produced (most of them in the F version), only a few hundred remained in service after the end of World War II. However, these too were soon withdrawn from service. Technical data (for the B-17G): Maximum speed: 510 km / h, practical ceiling: 10,670 m, operating radius: 1,760 km, armament: fixed – 13 machine guns cal. 12.7 mm, suspended – up to 5800 kg, normally – 2742 kg bombs.