The XB-35 made its first flight on 25th June 1946, powered by four Pratt & Whitney R-4360 Twin Wasp Majors of 3,000 h.p. each, driving contra-rotating propellers. At one stage the aircraft was flown with temporary three-bladed co-axial propellers on the port outer engine.

The B-32 was developed from the B-24 to Superfortress standard, but was slowed down in production by difficulties of redesign. Remote control of the armament was abandoned, and so was pressurisation. Eventually, production continued only in order to cover possible B-29 setbacks, and stopped with the end of the war. Span was 135 ft., armament ten .50-in. machine guns, and motors were Wright R3350-23s.

Basically a bomber conversion of the DC-3, the Douglas DB-1 came first in the Bomber Competition of 1936; 133 were ordered as B-18, powered by Wright Cyclone R-1820-15. The type underwent many engine and internal changes. Version shown is an RB-18C.

This interesting picture of an RB-34 shows a machine with R.A.F. camouflage and serial (AJ311), and U.S.A.A.F. stars and bars. The Lockheed Vega 37 was built under British contract as the Ventura I. The B-34 contract for the U.S. included many transferred to the R.A.F. as Ventura II and IIA.

The Martin Model 139 was produced in many forms and, like the Keystones, they differed mostly in engines. The B-10B illustrated was the standard U.S.A.A.C. type with Wright Cyclone SGR-1820-G9s of 800 h.p.

The 35-ton, 150-ft.-span XB-15 first flew on 15th October 1937. In 1939, as part of the Air Corps' 30th Anniversary Celebrations, it broke the weight-to-height record held by Russia. Armament was fitted in nose and dorsal turrets, and two lateral and two ventral blisters. The Pratt & Whitney SC-G motors, of 1,050 h.p. each, could be reached in flight through the wing.

Like many other early U.S. bombers. the Y1B-7 illustrated here began life in another department; in this case as the XO-36. Although tested for the service, this 65-ft.-span, four-man crew bomber was never produced in quantity.

Very little of the B-18 except wings and power plants was recognisable in the B-23. Span was 92 ft. Wright Cyclone R-2600-3s of 1.350 h.p. each were originally fitted, with constant-speed, fully-feathering airscrews, while later machines had the GR-2600-A5B of 1.600 h.p. The aircraft shown here has a solid nose in place of the original glazed bombardier's panels.

The XB-43 was a Mixmaster with the Allison V-1710-125s replaced by two General Electric TG-180 (J-35-GE-3) turbojets developing 4.000-lb. static thrust each.

The Keystone B-3As were all transferred from the Light Bomber class (LB-10), having their Wright R-1750s changed for Pratt & Whitney Hornets. The gunner - visible in the photograph - looked after twin Lewises in his cockpit and a Single one firing through a trap in the floor.

The majority of the Keystone bombers were - externally - almost identical, the only differences being in engines. In the absence of a serial it is difficult to identify this photo accurately but it is almost certainly a B-6A with Wright Cyclone SGR-1820-1 motors.

Only one Y1B-17A was built, but it revolutionised air warfare, for the G.E. turbo-superchargers on its Wright G Cyclones made possible bombing above 30,000 ft. It flew first in January 1939, and in August carried an 11,023-lb. payload at an average 259 m.p.h. It was intended to test the Y1B-17A to destruction, but the accidental spinning and recovery of a Y1B-17 provided the data on stresses instead, and the -A was converted for high-altitude tests. Photo shows camouflage used in 1940 War Games.