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.