B47stratojetAssoc history Chapter 9

The early versions of the Stratojet were all severely underpowered, and in pursuit of a more powerful Stratojet, the Air Force proposed a version of the B-47 powered by four 10,090 lb. static thrust Allison J71-A-5 turbojets. The designation YB-56 was assigned to this project, since the different engine configuration was thought to justify a change in designation. A reconnaissance version known as RB-56A was also planned. According to the original planning, the 88th B-47B (50-092) was scheduled be converted to YB-56 configuration as a testbed for the concept.

For a time, the YB-56 was intended to be the "definitive" Stratojet. However, since the airframe was basically that of a "stock" B-47B, the designation of the YB-56 was changed to YB-47C. The J71 engine was later found to be unsuitable for the Stratojet, and a decision was made to switch to the new Pratt and Whitney YJ57 turbojet. However, these engines were not yet available, and, in any case, they were already earmarked for the B-52. Consequently, the YB-47C program was canceled in December of 1952, and no four-jet Stratojet was ever built.