Fairey Battle was merely obsolete. Boulton Paul Defiant was a bad idea.
Good distinction, though I'd say that maybe the idea for the Defiant was obsolete...?
Another candidate: the intended replacement for the very successful B5N "Kate" carrier-based torpedo bomber, the B6N Tenzan/"Jill" ? Thing was in the testing and development phase for a few years, had structural deficiencies (weak tailhook, weak main gear), handled poorly (unstable in yaw), and always was difficult to fly at low speeds (i.e., landing)...