Northrop N-55: the tailless patroller from Hawthorne

In late 1948, the US Air Force concluded that its ability to defend the continental US from airstrikes was inadequate. Thus, it promulgated 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. After becoming aware of this situation, the Northrop company proposed an all-wing 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 bomber project 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. Search radars operating in the X-band or S-band ranges would be internally accommodated in the nose and tail of the N-55. 

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

Two N-55 variants were studied. The baseline N-55 design was powered by two Northrop XT37-NA-3 turboprops each delivering 10,000 hp, and it was 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). The N-55A had the same wingspan but was powered by 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.

References:

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

The new and improved B-49: the Northrop N-40

In 1948 Northrop began to undertake design work on an improved version of the YB-49 prototype flying wing jet bomber, bearing the mind the fact that the YB-49 had some serious deficiencies in operating range and payload capability. For example, the YB-49's operating range was more or less on par with that of the B-47 Stratojet, and its bomb bay was not big enough to carry free-fall nuclear bombs. On August 23, 1948, the US Air Force, seeing that it needed a second flying example of the B-49 bomber, modified the recently signed contract for 30 RB-49s with an amendment to convert one of the RB-49s into a B-49.

Three-view drawing of the Northrop N-40 with six Westinghouse J40 turbojets from the Northrop company documents. 

Northrop's proposal for improvising the YB-49 design was given the company designation N-40. It would have the same airframe as the YB-49 but feature a new center bomb bay in the crew nacelle that measured 170 inches in length, 80 inches wide, and 80 inches deep, big enough to carry America's large-diameter nuclear weapons. To improve bombing accuracy, the N-40 utilized an AN/APQ-24 bombing/navigation system on the lower aft portion of the crew nacelle, and a Minneapolis-Honeywell E-7 autopilot system was incorporated into the design to ameliorate the phugoid oscillations noticed during flight tests of the YB-49. Twin and quad 0.50 caliber machine gun turrets were considered for installation, and one design iteration had one remotely controlled turret below the starboard nose area outboard of the bombardier's glass and one remotely controlled turret in the aft upper fuselage, both with four 0.50 caliber machine guns. The baseline N-40 design had eight 4.850 lb (21.57 kN) thrust General Electric J47-GE-3 turbojets, six buried in the wing and two in pods below the wings. Northrop also looked an N-40 design with six wing-embedded 7,500 lb (33,36 kN) thrust Westinghouse J40 turbojets, and an option to fit the N-40 with six 6,250 lb (27.8 kN) thrust Pratt & Whitney J48s (license-built version of the Rolls-Royce Tay turbojet) was also considered. Another alternate engine provision explored for the N-40 involved the use of two Turbodyne T37 turboprops in conjunction with two or four J40s, similar to the powerplant arrangement planned for the Northrop N-37B (RB-49C).

Although Northrop hoped that the N-40 would entice the US Air Force to order more examples of the B-49, the N-40's life was destined to be short-lived. The cancellation of the RB-49 contract in January 1949 put any thought of clearing the N-40 for full-scale development to rest, and the N-40 was not proceeded with. Given that the N-40 would have had potentially greater range than the YB-49 and the ability to carry free-fall nuclear bombs, it might have been a serious competitor to the B-52 had it been built. 

References:

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