I doubt the chin turret of the YB-40 could hold 6 50cal machine guns. From all accounts I've read, it held only two. But, the chin turret was adopted by the B-17G to help counter frontal attacks. The main problem with the YB-40, aside from being underpowered, was accuracy. The guns already had a huge dispersion on regular B-17s. Combine that with the massive vibration of multiple guns firing on a YB-40 and the problem multiples itself.
