PBJ-1J / B-25J-NC. Used to launch the "Tiny Tim" 11.75" rocket.

The XB-28 (North American model NA-63) was originally conceived as a high altitude version of the B-25 medium bomber. The resulting design, while maintaining the same overall configuration of the Mitchell (with the exception of a single vertical tail instead of B-25's twin tail), was visually more reminiscent of the B-26 Marauder. Other differences in the XB-28 design included a pressurized fuselage and three remote-controlled twin .50 caliber gun turrets (upper, lower, and tail). Two prototypes were ordered in February 1940, with the first flight taking place on April 26, 1942. After testing of the two prototypes, the project was canceled with no further examples being built. The reason for this, according to the USAF archives, was that "although the XB-28 was a successful design, the aircraft never went into production. One reason for this was high altitude bombing was too susceptible to errors caused by wind, cloud cover, etc. especially in the Pacific Theater of Operations. Another was the increasing effectiveness of medium bombers at low and medium levels along with improved tactics. Finally, the performance gains were not considered great enough to interrupt production of proven combat models." The second prototype XB-28 was built as a high altitude photo reconnaissance aircraft, and was designated XB-28A, but like the first XB-28 prototype it was the only one of its kind.