Little-known Northrop flying wings of 1940s-early 1950s, part 2: N-31 medium bomber and N-55 patrol flying wing designs

In the first part of a trio of posts discussing little-known Northrop designs for large flying wing aircraft, I discussed in detail Northrop's scheme to convert a YB-35 airframe under construction into a testbed for the Northrop XT37-NA-3 turboprop engine under the designation EB-35B, considering how Northrop touted the EB-35B as potentially leading to a fuel-efficient gas turbine-powered B-35 derivative with greater range than the jet-powered B-49. However, even though the United States Air Force cancelled the B-49 program in 1949, the Northrop Corporation continued to look into designs for advanced flying wing bombers in hopes of remedying the shortcomings of the jet-powered YB-49.

The first advanced Northrop combat flying wing from the 1940s-early 1950s interval that is worthy of discussion is the N-31 medium bomber. Even before the second YB-49 crashed over Muroc Dry Lake in June 1948, Northrop was starting to realize that it would have to start anew with novel flying wing bomber designs that could theoretically correct problems inherent in earlier Northrop flying wings like the B-35 and B-49 if it hoped to convince the US Air Force of the viability of flying wing bombers. With the Soviet Union having detonated its first nuclear weapon in present-day Kazakhstan in August 1949, Northrop was eager to develop a flying wing bomber with a bomb bay big enough to carry America's available nuclear weapons and ultimately the in-development hydrogen bomb. Multiple flying wing bomber and reconnaissance designs with either turbojet or turboprop engines were investigated under the N-31 company designation with two, four, or six engines.

3-view drawing of the Westinghouse J40-powered N-31, with accompanying specification at the top right corner

The first flying wing bomber design studied under the N-31 label was a strategic bomber powered by six Westinghouse J40-WE-6 turbojets (each with 7,500 pounds of thrust), conceived in February 1947. It was similar to the YB-49, YRB-49A, RB-49, and EB-35B in featuring vertical stabilizers on the trailing edge of the wing, but differed in having a blended wing body layout with a forward fuselage section reminiscent of the delta-wing Avro Vulcan bomber. The arrangement of the turbojets in the turbojet-powered N-31 design iteration was quite unorthodox; four of the J40s were buried between the center section of the aircraft and the vertical stabilizers, but the other two were placed on the wingtips. The jet N-31 design had a wingspan of 128 feet 4 inches (39.11 meters) and a length of 74 feet 11 inches (22.83 meters), with a crew of five and provisions for twin 20 mm cannons (0.50 caliber) in nose and tail barrel turrets. It was intended to have a combat radius of 2,022 miles (3,254 km) with a 10,000-pound bomb load, a cruising speed of 502 mph (808 km/h), and a top speed of 608 mph (978 km/h).

3-view drawing from Northrop company documents of N-31A design with two Allison T40 turboprop engines. Note the absence of upright vertical stabilizers.

The N-31A design (envisaged January 1948) had the same wingspan, length, armament, gross weight, and wing sweep and baseline N-31, but returned to some aspects of the design layout of the XB-35, especially the lack of vertical stabilizers, and it was powered by two 7,500 hp Allison T40 turboprops, possessing an operational range of 2,600 miles (4,441 km) with a 10,000-pound bomb load. The N-31B, would have been similar to the baseline N-31 but with an all-wing configuration and just the four wing-mounted turbojets, while the N-31C proposed in late April 1948 would have had two Turbodyne T37s, and the N-31F was to be a a photo-reconnaissance derivative of the N-31B (see Buttler 2010, pp. 19-20, for more details). In May 1950, two additional N-31A proposals were conceived, one with two 10,000 hp Turbodyne T37 turboprops each driving six-bladed counter-rotating propellers, a cruising speed of 500 mph (804 km/h), a top speed of 518 mph (833 km/h), a range of 2,760 miles (4,441 km) when unrefueled or 5,580 miles (8,980 km) when refueled, an empty weight of 161,540 pounds (73,227 kg), a gross weight of 222,710 pounds (101,019 kg), and a maximum altitude of 43,000 feet (13,106 meters), and another with four T40s each driving six-bladed counter-rotating propellers, an empty weight of 175,400 pounds (79,560 kg), a gross weight of 212,000 pounds (96,206 kg), a cruising speed of 506 mph (814 km/h), and an altitude of 37,000 feet (11,277 meters) (Rose 2010, p. 84). The combat radius of the T40-powered design when unrefueled was similar to that of the T37-powered proposal but had lower range with in-flight refueling were lower (4,027 miles for Allison-powered version vs. 5,580 miles for T37-powered design). 

Northrop N-55 (left) and N-55A (right) design studies for a long-range patrol flying wing

Northrop's study of advanced combat flying wings in the 1940s and early 1950s was not confined to bomber aircraft. Prior to the cancellation of the RB-49, Northrop looked at the potential of the flying wing to be used for long-range patrol missions. With the US Air Force promulgating an across-the-board scheme to improve American air defense systems, which partly led to the WS-201 (aka 1954 Interceptor) requirement that resulted in development of the delta-winged Convair F-102 Delta Dagger and F-106 Delta Dart as well as the trisonic Republic XF-103 Thunderwarrior, the Northrop company adapted the YB-49 airframe into a design for a patrol plane under the designation N-55. The wingspan of the N-55 would be same as that the B-35 and B-49, but the fuselage design would mirror that of the N-31 flying wing designs in utilizing fore and aft crew nacelle extensions, giving the plane a length of 61.8 feet (18.8 meters). The N-55 concept was basically not just intended for long-range patrol missions but also would serve to function as an airborne radar picket ship that could fire air-to-air missiles against enemy planes that intruded into North American airspace. As noted by Chong (p. 66), search radars operating in the X-band or S-band ranges would be internally accommodated in the nose and tail of the N-55. Two N-55 variants were studied, the baseline N-55 powered by two Northrop XT37-NA-3 turboprops each delivering 10,000 hp and armed with 44 AAM-N-2 (later GAR-1/2/3/4/5/6/9 and finally AIM-4) Falcon air-to-air missiles (of which a dozen would be fired from leading edge tubes with eyelid covers, the rest being housed in two weapons bays), and the N-55A using six Westinghouse J40 turbojets. The turboprop-powered N-55 looked like a cross between the baseline N-31 and later N-31A versions because it had two T37 turboprop engines and a pair of vertical stabilizers, while the engine arrangement of the N-55A resembled that of the YB-49, with the jet engines buried between two pairs of vertical stabilizers on the wing's trailing edge.

But the N-31 and N-55 flying wing studies were all for naught, despite offering potential remedies to the stability and range problems that made the YB-49 unsuitable as a bombing platform and ineligible to carry large nuclear and thermonuclear weapons. Continued concerns about longitudinal instability of flying wings combined with evolving air defense priorities meant that the N-31 and N-55 were never green-lighted for full-scale development by the US Air Force, so they remained paper projects only.
   

References:

Buttler, T., 2010. American Secret Projects: Bombers, Attack, and Anti-Submarine Aircraft 1945-1974. Hersham, UK: Ian Allan Publishing.

Chong, T., 2016. Flying Wings & Radical Things: Northrop's Secret Aerospace Projects & Concepts 1939-1994. Forest Lake, MN: Specialty Press.

Rose, B., 2010. Secret Projects: Flying Wings and Tailless Aircraft. Hersham, UK: Ian Allan Publishing.