Development of an improved B-29 started in 1944, with the desire to replace the unreliable Wright R-3350 Duplex-Cyclone engines with the more powerful four-row, 28-cylinder Pratt & Whitney R-4360 Wasp Major radial engine, America's largest-ever displacement aircraft piston engine in large-scale production.
A B-29A-5-BN (serial number 42-93845) was modified by Pratt & Whitney as a testbed for the installation of the R-4360 in the B-29, with four 3,000-horsepower (2,200 kW) R-4360-33s replacing the 2,200-horsepower (1,600 kW) R-3350s.
The modified aircraft, designated XB-44 Superfortress, first flew in May 1945.

The planned Wasp-Major powered bomber, the B-29D, was to incorporate considerable changes in addition to the engine installation tested in the XB-44.
The use of a new alloy of aluminum, 75-S rather than the existing 24ST, gave a wing that was both stronger and lighter, while the undercarriage was strengthened to allow the aircraft to operate at weights of up to 40,000 pounds (18,000 kg) greater than the B-29.
A larger vertical fin and rudder (which could fold to allow the aircraft to fit into existing hangars) and enlarged flaps were provided to deal with the increased engine power and weight, respectively.

Armament was similar to that of the B-29, with two bomb bays carrying 20,000 pounds (9,100 kg) of bombs, and a further 8,000 pounds (3,600 kg) externally.
Defensive armament was 13 × 12.7mm (.50 inch) machine guns (or 12 machine guns and one 20 mm (0.8 in) cannon) in five turrets.

First flying in May 1945, the sole XB-44 proved 50–60 mph (80–100 km/h) faster than the standard B-29, although existing sources do not indicate how much of this increased speed was due to differing aircraft weight due to deleted armament or increased power due to the R-4360-33 engines.

An order for 200 B-29Ds was placed in July 1945, but the ending of World War II in August 1945 prompted mass cancellations of outstanding orders for military equipment, with over 5,000 B-29s canceled in September 1945.
In December that year, B-29D orders were cut from 200 to 60, while at the same time the designation of the aircraft was changed to B-50.

Officially, the aircraft's new designation was justified by the changes incorporated into the revised aircraft, but according to Peter M. Bowers, a long-time Boeing employee and aircraft designer, and a well-known authority on Boeing aircraft, "the re-designation was an outright military ruse to win appropriations for the procurement of an airplane that by its B-29D designation appeared to be merely a later version of an existing model that was being canceled wholesale, with many existing examples being put into dead storage."

The first production B-50A (there were no prototypes, as the aircraft's engines and new tail had already been tested) made its maiden flight on 25 June 1947, with a further 78 B-50As following.
The last airframe of the initial order was held back for modification to the prototype YB-50C, a planned version to be powered by R-4360-43 turbo-compound engines.
It was to have a longer fuselage, allowing the two small bomb bays of the B-29 and the B-50A to be replaced by a single large bomb bay, more suited to carrying large nuclear weapons.
It would also have longer span wings, which required additional outrigger wheels to stabilize the aircraft on the ground.

Orders for 43 B-54s, the planned production version of the YB-50C, were placed in 1948, but the program was unpopular with Curtis LeMay, commander of Strategic Air Command (SAC), as being inferior to the Convair B-36 Peacemaker and having little capacity for further improvement, while requiring an expensive redevelopment of air bases owing to the type's undercarriage.
The B-54 program was therefore canceled in April 1949, work on the YB-50C being stopped prior to it being completed.

While the B-54 was canceled, production of less elaborate developments continued as a stopgap until jet bombers like the Boeing B-47 and B-52 could enter service.
Forty-five B-50Bs, fitted with lightweight fuel tanks and capable of operating at higher w
Development of an improved B-29 started in 1944, with the desire to replace the unreliable Wright R-3350 Duplex-Cyclone engines with the more powerful four-row, 28-cylinder Pratt & Whitney R-4360 Wasp Major radial engine, America's largest-ever displacement aircraft piston engine in large-scale production. A B-29A-5-BN (serial number 42-93845) was modified by Pratt & Whitney as a testbed for the installation of the R-4360 in the B-29, with four 3,000-horsepower (2,200 kW) R-4360-33s replacing the 2,200-horsepower (1,600 kW) R-3350s. The modified aircraft, designated XB-44 Superfortress, first flew in May 1945.
The planned Wasp-Major powered bomber, the B-29D, was to incorporate considerable changes in addition to the engine installation tested in the XB-44. The use of a new alloy of aluminum, 75-S rather than the existing 24ST, gave a wing that was both stronger and lighter, while the undercarriage was strengthened to allow the aircraft to operate at weights of up to 40,000 pounds (18,000 kg) greater than the B-29. A larger vertical fin and rudder (which could fold to allow the aircraft to fit into existing hangars) and enlarged flaps were provided to deal with the increased engine power and weight, respectively.
Armament was similar to that of the B-29, with two bomb bays carrying 20,000 pounds (9,100 kg) of bombs, and a further 8,000 pounds (3,600 kg) externally. Defensive armament was 13 × 12.7mm (.50 inch) machine guns (or 12 machine guns and one 20 mm (0.8 in) cannon) in five turrets.
First flying in May 1945, the sole XB-44 proved 50–60 mph (80–100 km/h) faster than the standard B-29, although existing sources do not indicate how much of this increased speed was due to differing aircraft weight due to deleted armament or increased power due to the R-4360-33 engines.
An order for 200 B-29Ds was placed in July 1945, but the ending of World War II in August 1945 prompted mass cancellations of outstanding orders for military equipment, with over 5,000 B-29s canceled in September 1945. In December that year, B-29D orders were cut from 200 to 60, while at the same time the designation of the aircraft was changed to B-50.
Officially, the aircraft's new designation was justified by the changes incorporated into the revised aircraft, but according to Peter M. Bowers, a long-time Boeing employee and aircraft designer, and a well-known authority on Boeing aircraft, "the re-designation was an outright military ruse to win appropriations for the procurement of an airplane that by its B-29D designation appeared to be merely a later version of an existing model that was being canceled wholesale, with many existing examples being put into dead storage."
The first production B-50A (there were no prototypes, as the aircraft's engines and new tail had already been tested) made its maiden flight on 25 June 1947, with a further 78 B-50As following. The last airframe of the initial order was held back for modification to the prototype YB-50C, a planned version to be powered by R-4360-43 turbo-compound engines. It was to have a longer fuselage, allowing the two small bomb bays of the B-29 and the B-50A to be replaced by a single large bomb bay, more suited to carrying large nuclear weapons. It would also have longer span wings, which required additional outrigger wheels to stabilize the aircraft on the ground.
Orders for 43 B-54s, the planned production version of the YB-50C, were placed in 1948, but the program was unpopular with Curtis LeMay, commander of Strategic Air Command (SAC), as being inferior to the Convair B-36 Peacemaker and having little capacity for further improvement, while requiring an expensive redevelopment of air bases owing to the type's undercarriage. The B-54 program was therefore canceled in April 1949, work on the YB-50C being stopped prior to it being completed.
While the B-54 was canceled, production of less elaborate developments continued as a stopgap until jet bombers like the Boeing B-47 and B-52 could enter service. Forty-five B-50Bs, fitted with lightweight fuel tanks and capable of operating at higher w
Boeing B-50 Superfortress