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 R-4360-33s replacing the R-3350s. The modified aircraft, designated XB-44 Superfortress, first flew in May 1945.