Six B-29s were fitted with R-3350-CA-2 fuel injection engines and the revised "Andy Gump" nacelles that were intended for late production B-29As in a program to service test these new engine installations. These aircraft were redesignated YB-29J.
Some of these YB-29Js were used for photo-reconnaissance work as RB-29J. Two of them (44-86398 and 44-86402) were converted to aerial tankers under the designation YKB-29J as part of a test of the Boeing-developed boom aerial refuelling system that was introduced in the KB-29P program. However, these two aircraft retained their service test and J-series designations.