Northrop YB-35

The Northrop YB-35, Northrop designation N-9 or NS-9, was an experimental heavy bomber aircraft developed by the Northrop Corporation for the United States Army Air Forces during and shortly after World War II. The airplane used the radical and potentially very efficient flying wing design, in which the tail section and fuselage are eliminated and all payload is carried in a thick wing. Only prototypes and pre-production aircraft were built, although interest remained strong enough to warrant further development of the design as a jet bomber, under the designation YB-49.

The B-35 was the brainchild of Jack Northrop, who made the flying wing the focus of his work during the 1930s. In 1941, before the United States entered World War II, Northrop and Consolidated Vultee Corporation had been commissioned to develop a large wing-only, long-range bomber designated XB-35 and XB-36 respectively. Northrop advocated a "flying wing" as a means of reducing parasitic drag and eliminating structural weight not directly responsible for producing lift. Consolidated Vultee proposed a more conventional design with fuselage and tail, which was much larger and heavier. In theory, the B-35 could carry a greater payload faster, farther, and cheaper than a conventional bomber.

In December 1941, the Army Air Forces awarded prototype contracts to both Northrop and Consolidated Vultee for a bomber that could carry 4,500 kg (10,000 lb) of bombs to a round-trip mission of 16,000 km (10,000 mi). Requested performance was a maximum speed of 720 km/h (450 mph), cruise speed of 443 km/h (275 mph), and service ceiling of 14,000 m (45,000 ft). This aircraft would be able to bomb Nazi-occupied Europe in the event that Britain fell (this was similar to Nazi Germany's own Amerikabomber program design competition through the RLM, itself initiated in the spring of 1942). The original April 1941 USAAC proposal was first submitted to Boeing and Consolidated Aircraft Company and led to the production of the Convair B-36. In May, one month before the USAAF was created, the contract was also extended to include Northrop, inviting it to submit a design along the lines it was already exploring.

Since the new aircraft would require a significant amount of engineering work in untested waters, the first order placed was actually for two prototypes of the XB-35, and included Northrop's plan to also build two all-wood one-third scale flying models to measure performance and stability; these were dubbed the Northrop N-9M (M standing for model). This aircraft would be used to gather flight test data on the Flying Wing design. Jack Northrop also hired part-time the leading aeronautical designer of the day Theodore von Kármán, to evaluate and who approved of Northrop's initial design, and to start building the tooling for building the prototypes ... as explained in detail in the book "Goodbye Beautiful Wing" by Terrence O'Neill (ISBN 978-0979012129).^{[citation needed]} The N9Ms would also be used as a flight trainer, to familiarize pilots with the radical, all-wing concept.

Early in 1942, design work on the XB-35 itself began in earnest. Unlike conventional aircraft, truly "tailless" flying wings do not have a rudder for lateral control, so a set of clamshell-like, double split flaps (so-called flaperon, a portmanteau of flap and aileron) on the trailing edge of the wingtips were used. When aileron control was input, they were deflected up or down as a single unit, just like an aileron. When rudder input was made, the two surfaces on one side opened, top and bottom, creating drag, and yawing the aircraft. By applying input to both rudder pedals, both sets of surfaces were deployed creating drag so that the airspeed or the glide angle could be manipulated.

On 22 November 1941, the United States Army Air Forces (USAAF, or AAF) signed the development contract for an XB-35; the contract included an option for a second aircraft, which was exercised on 2 January 1942. The first was to be delivered in November 1943, the second in April of the next year.

Detailed engineering began in early 1942. A fuselage-like crew cabin was to be embedded inside the wing; it included a tail cone protruding from the trailing edge. This tail cone would contain the remote sighting stations for the bomber's gunners and a cluster of rear-firing machine guns in the production aircraft. In the midsection of the cabin, there were folding bunks for off-duty crew on long missions. The aircraft's bomb load was to be carried in six smaller bomb bays, three in each wing section, fitted with roll-away doors; this original design precluded the carrying of large bombs, and the early atomic bombs, without bomb bay redesign and modifications. Production aircraft would have defensive armament of twenty 0.5 in (12.7 mm) machine guns or 20 mm cannon, carried in six turrets, two turrets along the aircraft's center line, four above and below the outer wings, and four in the "stinger" tail cone. The B-35 would take advantage of a new aluminum alloy devised by Alcoa; it was considerably stronger than any alloy used previously.

In June 1946, the XB-35 made its first flight, a 45-minute trip from Hawthorne, California, to Muroc Dry Lake, without incident. The XB-35's engines and propellers were AAF property and had not been tested for engine-propeller compatibility by either Pratt & Whitney, Hamilton Standard, or by the AAF which bought them at Wright Field without testing them or assuring reliability, and then shipped them to Northrop. Microfilmed records of reports and correspondence of the XB-35 program relate that after three or four flights, power plant and propeller vibrations increased, and the very efficient contra-rotating propellers began failing with frustrating frequency. Meetings were called by Northrop, of the AAF, Pratt & Whitney and Hamilton Standard where no one would take responsibility for correcting the AAF's engines and propellers. In addition, the AAF failed to supply the AC electrical alternator, insisting on Northrop using an onboard auxiliary power unit (APU) unit driven by an automotive engine which limited the high-altitude, high-speed XB-35 to test flights below 15,000 ft (4,600 m). The AAF also refused to allow Northrop proposed modification of the bomb bays to carry the standard Mk. 3 atomic bomb, while at the same time declaring the AF would not buy the bomber unless it could carry the A-bomb. Northrop reluctantly agreed to try a single-rotation propeller, which slightly increased takeoff distance and a reduced rate-of-climb and maximum speed.