Northrop B-35

Last revised January 19, 2000




John K. Northrop was one of the pioneering giants in aviation. He got his start in aviation back in 1916, when he went to work for the Loughead Aircraft Manufacturing Company. Jack Northrop joined Douglas Aircraft in 1923, and rose to the rank of chief engineer, but he later left Douglas to rejoin his old team at what was now known as Lockheed. While there, he worked on the Lockheed Vega monoplane. In 1928, Northrop helped found the Avion Corporation, which built the well known Alpha monoplane. However, in 1930 he was forced to sell Avion to the United Aircraft and Transport Corporation.

At the height of the Depression in 1932, with partial financing from the Douglas Aircraft Company, Jack Northrop founded the first aircraft company bearing his name. His company built the famous Gamma and Delta monoplanes which were so successful during the 1930s. However, in August of 1939, Jack Northrop left the company he had founded, since by that time it had become just another division of Douglas, and founded an entirely new and completely independent company named Northrop Aircraft, Inc at Hawthorne, California.

As early as 1923, Jack Northrop had been convinced that the flying wing, in which the aircraft carried all loads and controls within the wing and dispensed with fuselage and tail sections, was the next major step forward in aircraft design. In support of the flying wing idea, Jack Northrop had built a number of small-scale demonstrators to evaluate the concept. One of these was the N1M flying wing demonstrator which flew for the first time on July 3, 1940.

On April 11, 1941, the USAAF issued a request for proposals for a high-altitude bomber that could carry a 10,000-pound bombload halfway across a 10,000 mile range. Maximum speed was to be 450 mph at 25,000 feet, cruising speed 275 mph, service ceiling 45,000 feet, and a maximum range of 12,000 miles at 25,000 feet. Invitations for preliminary design studies were sent to the Consolidated Aircraft Corporation and to the Boeing Airplane Company. The Consolidated submission was eventually to emerge as the B-36. As part of this project, Northrop was contacted on May 27, 1941 and asked to provide studies of a flying wing proposal as it related to requirements for a range of 8000 miles at 25,000 feet with one ton of bombs, a cruising speed of 250 mph, a service ceiling of 40,000 feet, and a bombload of 10,000 pounds, which were much less demanding than those of the April ll RFP.

In August of 1941, slightly more ambitious requirements were again submitted to Northrop. The flying wing bomber project (designated NS-9 by the company) received approval for an initial start from the USAAC in September of 1941, following a visit to the Northrop plant by Assistant Secretary of War Robert Leavitt, General Henry H. Arnold, and Major General Oliver P. Echols. The order was confirmed on October 30, 1941. The contract included a purchase order for engineering data, model tests, plus a 1/3-scale flying mockup known as the N9M. On November 22, a contract for a single XB-35 prototype and an option for a second was signed. The option for the second XB-35 was exercised on January 2, 1942. According to the terms of the contract, the first XB-35 was to be delivered in November 1943, with the second following in April of 1944.

Detailed design work on the XB-35 began in early 1942, and the XB-35 full-scale mock-up was approved on July 5, 1942. On December 17, 1942, 13 YB-35 service test aircraft were ordered.

Two more N-9M flying scale models were ordered in early 1943, with a fourth being ordered in mid-1943.

The advantages of a flying wing format were perceived as providing both low drag and high lift, which meant that the XB-35 could carry any weight faster, farther, and cheaper than conventional aircraft. In addition, the use of a flying wing meant that simpler construction methods could be used with fewer structural complications. A flying wing should cost less to build since it was built as a single unit with no added tail or fuselage. A flying wing provides a better weight distribution for the offensive load, since compartments along the entire span could distribute the weight of the bomb load much more evenly. Finally, a flying wing presented a smaller target when seen from fore, aft, or from the side when engaged in either offensive or defensive operations.

Northrop lacked adequate production facilities for the manufacture of production B-35s, and enlarging the company's Hawthorne plant was out of the question at that time. At the end of 1942, in search of more production facilities, Northrop began negotiating with the Glenn L. Martin Company. Northrop management indicated that they would be satisfied to fabricate only the XB-35s and the YB-35s, with Martin having the responsibility for building the production B-35s at its plant in Baltimore, Maryland. A contract for 200 B-35s was initially planned in November of 1942, and was formally issued on June 30, 1943. The first production B-35 was to be delivered by June of 1945.

In support of the program, Northrop built four 60-foot wingspan (about one-third the size of the proposed B-35) N9M flying wing test aircraft to train pilots in handling flying-wing aircraft and to see if the general concept was feasible. They were of mixed wood and metal construction, with the center section being of welded steel tubing. The covering was of wood and metal panels, with the outer wing panels being of wood with metal wing slots and wing tips. The four N9Ms were later called N9M-1, N9M-2, N9M-A, and N9M-B respectively. They were initially powered by a pair of 290 hp Menasco C65-4 six-cylinder air-cooled engines each driving a pusher two-bladed propeller by means of an extension shaft via a fluid-drive coupler. The engines were cooled by air admitted by large under-wing scoops. The N9M-B was later fitted with two air-cooled 400 hp Franklin engines. Provisions were made for a pilot and one passenger, both housed underneath a single transparent bubble canopy. It was provided with a retractable tricycle landing gear, and a rear outrigger tail wheel was fitted. These 4 aircraft flew with neither civil registrations nor military serials. The design details worked out in the N9M were incorporated into the design of the XB-35.

The first N-9M flew for the first time on December 27, 1942. It crashed on May 19, 1943, killing its pilot. On the maiden flight of the second model on June 24, 1943, the cockpit canopy of the aircraft flew off while in flight, but the pilot was able to land successfully. Nearly all the flight tests of the N-9M were shortened by mechanical failures of one kind or another, particular with failures in the Menasco engines. The fourth and last N9M (the N9M-B) flew for the first time on September 21, 1943.

The N9M-B (the last of the four) managed to survive all these years, and was restored to flying condition over a period of 12 years by volunteers at the Chino 'Planes of Fame Museum' and flew again, for the first time after about 45 years, on November 11, 1994. The new civil registration of the N9M-B is 'N9MB'.

The wingspan of the B-35 was 172 feet, and the leading edges were swept back at an angle of 27 degrees. The wing of the B-35 was 37 1/2 feet wide at the center, tapering to 9 feet wide at the tips. Because of the wing sweep, the overall length of the aircraft was slightly over 53 feet.

The lateral control that was normally provided by conventional rudders was provided on the B-35 by a set of double split flaps located on the trailing edges of the wingtips. These operated by having the split flaps open up in butterfly fashion to provide a braking effect. When the left rudder pedal was depressed, the left flaps would open up, forcing a turn to the left. If both pedals were depressed, both split flaps would open up to increase the gliding angle or reduce the air speed. These double split flaps could also act as trim flaps, and could be adjusted as a unit either up or down to trim the airplane longitudinally.

Elevons were located along the trailing edge of each wing inboard of the trim flaps. When deflected together in the same direction (by the pilot moving the control column fore or aft), they could cause the airplane to descend or climb. When operated differentially (by having the pilot move the control wheel left or right), they caused the airplane to bank left or right in a fashion similar to the function of conventional ailerons. For landings and takeoffs, A set of flaps were located in the wing trailing edge near the center.

The aircraft was powered by four Pratt & Whitney Wasp Major air-cooled radials (two R-4360-17s and two R-4360-21s) with double superchargers and feed by cooling air coming from long slots cut into the wing leading edge. Each engine drove a set of coaxial, counter-rotating four-bladed pusher propellers mounted at the end of a driveshaft that protruded beyond the trailing edge of the upper wing surface.

The B-35 was 20 feet tall when sitting on its tricycle landing gear. 5' 6" dual wheels on the main gear and a 4'8" wheel on the nose gear.

The crew of the XB-35 was carried in a crew cabin installed at the center of the wing, with a tailcone protruding beyond the central wing trailing edge. The normal crew was 9--a pilot, copilot, bombardier, navigator, engineer, radio operator and 3 gunners. The pilot sat in the very front of the wing center section (slightly offset to the left of center) underneath a transparent bubble-type canopy. The copilot sat to the right of the pilot and somewhat lower down, and sighted through a set of transparent windows cut into the front of the wing. His visibility, though, was fairly marginal. The bombardier's station was located to the right of the copilot's seat, and the bombardier operated the bombsight by aiming it through a square window cut into the forward underwing surface. The navigator and flight engineer sat to the rear of the copilot. The navigator had a small transparent bubble over his seat for the sighting of stars. Six more crew members could be added as substitutes on long-range missions, with folding bunks in the rear of the crew cabin to accommodate the off-duty crewmen.

The defensive armament was to consist of a set of remotely-controlled barbettes. A quartet of 0.50-inch machine guns were housed inside each of dorsal and ventral barbettes that were mounted on the tailcone along the wing's centerline. Four 0.50-inch machine guns were installed in the rear of the tailcone. A pair of 0.50-in machine guns were installed in each of four barbettes mounted on the wing outboard of the outermost engines, one above and one below the wing. The guns were remotely sighted by gunners sitting in stations in a bubble in the upper rear part of the tailcone, in a ventral station, and in a position in the pilot's bubble immediately behind the pilot's seat. The bombs were carried internally in eight individual bomb bays cut into the under surface of the wing outboard of the main crew cabin.

The XB-35 was built of an entirely new aluminum alloy developed by Alcoa. This alloy was considerably stronger than previous metals. The fuel was carried in self-sealing leak-proof fuel cells in the wing, and additional fuel could be carried in tanks in the bomb bay and in other wing compartment areas.

Unfortunately, by early 1944, the B-35 program was seriously behind schedule. Test results with the N-9M aircraft had indicated that the range of the XB-35 would most likely be 1600 miles shorter than anticipated. In addition, the maximum speed was estimated to be 24 mph slower than anticipated. Consequently, General Arnold began to question the wisdom of any extensive B-35 production program. In the meantime, the Martin company was experiencing severe shortages of trained engineers (many had been drafted) who could work on the B-35 project and had encountered delays in setting up the necessary tooling. These problems had forced Martin to push back the delivery date of the first B-35 to 1947. As a result, the USAAF concluded that it was unlikely that the B-35 would be ready in time to contribute to the war effort, and cancelled the Martin B-35 production contract on May 24, 1944.

However, this did not spell the death of the B-35 project, since the Air Technical Service Command felt that the XB-35 flying wing project was worthwhile for test purposes even if it never achieved operational status. In December of 1944, the USAAF decided that Northrop should go ahead and build the XB-35 and YB-35 aircraft as test vehicles. The first six of the YB-35s would be built on the XB-35 pattern, but with certain individual differences. On June 1, 1945, orders were issued to have two of the YB-35 airframes fitted with Allison J35-A-5 jet engines. The jet-powered flying wing was initially assigned the designation YB-35B, but this was later changed to YB-49. In 1945, after two more YB-35s had been added to the first YB-35 lot to replace the two that were earmarked for jet conversion, the USAAF told Northrop to manufacture the remaining 5 airplanes on the YB-35 contract to more advanced specifications, which resulted a redesignation to YB-35A.

The first XB-35 (serial number 42-13603) took off on its maiden flight on June 25, 1946, with Max Stanley as pilot and Dale Schroeder as flight engineer. On this first flight, the aircraft was flown from Hawthorne to Muroc Dry Lake, a flight lasting 45 minutes. Almost immediately, the flight test program ran into difficulties. Gear box malfunctions and propeller control difficulties caused the XB-35 to be grounded on September 11 after only 19 flights.

The second XB-35 (serial number 42-38323) took to the air for the first time on June 26, 1947. Only eight flights took place before Northrop was forced to ground this plane too.

The dual counter-rotating propellers and their gearboxes proved to be totally unsatisfactory, and both XB-35s had to be grounded in September of 1947 so that their dual-rotating propellers could be replaced by single-rotation propellers. Following the fitting of the new single-rotation propellers and the mounting of simpler gearboxes, flight testing of the first XB-35 was resumed in February of 1948. Seven more flights were made by the first XB-35 from February 12 to April 1, 1948. The new propeller installations operated without any particular mechanical difficulties, but there was considerable vibration and the performance of the aircraft was reduced.

The XB-35's intricate exhaust system was a maintenance nightmare, and by the middle of 1948 the cooling fans of the R-4360 engines were beginning to show signs of metal fatigue.

The first YB-35 (42-102366) flew on May 15, 1948, which was the only example actually fitted with defensive armament. The two XB-35s had carried only dummy turrets. The YB-35 was fitted with single-rotation propellers. This was destined to be the only one of the 13 YB-35s ordered that actually flew. Of the original 13 YB-35s ordered, four had been scheduled to be used as sources of spare parts for the extensive flight test program that was planned. However, by mid-1948 the piston-engined B-35 was definitely outdated, and the program was clearly doomed. A propeller-driven aircraft was simply much too slow for the era of jet propulsion, and the flying wing as it then existed was much too unstable to make a good bombing or camera platform. Nevertheless, the Air Force did not want to throw in the towel completely after having spent so much money, and for a while considered studying the feasibility of adapting the B-35 for the air-refuelling role, but this was not pursued any further.

By the end of 1948, it was planned for five YB-35s and 4 YB-35As to be converted to six-jet configuration and fitted with cameras and redesignated RB-35B (later to be redesignated YRB-49A). One YB-35 was earmarked for static testing, and another jet-converted YB-35A was to be fitted out as a test-bed for the Turbodyne T-37 turboprop engine, which was then under development. This test aircraft was to have been designated EB-35B (it was the last of the 13 prototypes) and would be capable of carrying two T-37 engines, although only one of these engines would actually be fitted initially. The second XB-35 was to have been fitted with a flexible-mount gear box to try and cure the problems with the vibrations in the single-rotation propellers.

In August of 1949, the two XB-35s and the first two YB-35s were scrapped. In November, the Air Staff cancelled plans for further conversions of YB-35s and YB-35As to jet propulsion. Scrapping of the remaining YB-35 airframes started in December of 1949 and was completed by March of 1950. The disassembly of the EB-35B testbed began in March of 1950. None of the series production B-35A were ever built.

Serials of Northrop B-35:

42-13603		Northrop XB-35
42-38323		Northrop XB-35
42-102366/102378	Northrop YB-35
				102367 and 102368 converted to YB-49. 
				102368 w/o 6-5-48
				102369/102375 scrapped before flying
				102376 converted to YRB-49A
				102377,102378 scrapped before flying

Specification of Northrop XB-35:

Engines: Two Pratt & Whitney R-4360-17, and two R-4360-21 Wasp Major air-cooled radials rated at 3000 hp each. Performance: Maximum speed 391 mph at 35,000 feet, cruising speed 183 mph. Service ceiling 39,700 feet. An altitude of 35,600 feet could be attained in 57 minutes. Range was 8150 miles at 183 mph with a 16,000 pound bombload, or 720 miles at 240 mph with 51,070 pounds of bombs. Dimensions: wingspan 172 feet 0 inches, length 53 feet 1 inches, height 20 feet 1 inches, wing area 4000 square feet. Weights: 89,560 pounds empty, 180,000 pounds gross, 209,000 pounds maximum. Armament: (only fitted to the first YB-35) Four 0.50-inch machine guns in remotely-controlled dorsal turret. Four 0.50-inch machine guns in remotely-controlled ventral turret. Four 0.50-inch machine guns in the rear of the tail cone. Two 0.50-inch machine guns in each of four barbettes installed above and below the wing outboard of the outermost engines. The guns were remotely sighted by gunners sitting in stations in a bubble in the upper rear part of the tailcone, in a ventral station, and in a position in the pilot's bubble immediately behind the pilot's seat. The bombs were carried in eight individual bomb bays cut into the under surface of the wing outboard of the main crew cabin.

Sources:


  1. American Combat Planes, Third Enlarged Edition, Ray Wagner, Doubleday, 1982.

  2. Northrop Flying Wings, Edward T. Maloney, World War II Publications, 1988.

  3. Post-World War II bombers, Marcelle Size Knaac, Office of Air Force History, 1988.