It has to be designed after the war began (even if it went into service after the war), and produced in quantity. One off prototypes do not qualify. It has to have failed miserably in it´s intended role and does not have any record in secondary roles (training, target towing etc.). I do have a couple in my head, but would like to see what others think.

Apart from being emediate pre-war design, the Botha really is a very good contender for your tight parameters Galdri, 580 built and completely useless in its torpedo bomber role (short, and dangerous use by 608 sqn only), and dangerous in its relegated training role.

The Warwick is another candidate, never being used in its intended bomber role, and being shoved around Coastal and Transport Commands to make use of the airframes.