The "porcupine" aircraft are loosely based on Project V-139 that turn 25 B-17Fs into YB-40 gunships. The gunships had an extra top turret, twin mount .50 cal machine guns in the waist positions and a chin turret under the the bomardier's position in the nose. Each gunship was heavily armored.
They were built by Vega and Douglas Aircraft. Some of these modified in England.
It proved to be a failure due to the fact the gunships were slower than B-17Fs after they dropped their bombs.
Some of the modifications were carried on in later models such as the chin turret.