Macchi M.C.91

The Macchi M.C.91 was a project of a medium bomber loosely based on the American Martin B-10 that, at the time, was a revolutionary design as it was the first bomber in the world with a completely sealed canopy.
Unlike the Martin B-10 which was made entirely out of metal, this was going to be made completely out of wood. The engine imposed by the Italian Regia Aeronautica was the Piaggio P.IX RC.40 radial engine rated at 630hp each of them which was an Italian version of the British-built Bristol Jupiter with a two-speed compressor which was expected to rate the engine at that horsepower we wrote earlier.
As the power of the engines was expected to be really low, engineer Mario Castaldi would have to cope with a wooden structure as light as possible and make that structure as aerodynamic as possible which, apparently, this design was the most aerodynamic of all his designs for the project. However, in order to achieve that goal, the payload had to be reduced with the payload hanging partially from outside of the fuselage. Therefore the configuration of the defensive armament, which was going to be of two 7,7mm Breda-SAFAT in a turret in the nose, another two 12,7mm Breda-SAFAT in a dorsal turret plus another 12,7mm one placed in a ventral hatch, was really complicated, as the front gunner and the dorsal one, were extremelly bulky and broke the aerodynamics and, furthermore, the position of the front gunner broke pilot's vision making take-offs and landing harder than usual.
Those so many problems leaded to the cancellation of the project.

Sources:
1. http://italianaircraftwwii.blogspot.com.es/2015/03/macchi-mc.html
2. https://www.secretprojects.co.uk/forum/index.php/topic,12121.msg118179.html#msg118179
3. http://www.aereimilitari.org/forum/topic/15374-concorso-1934-un-bombardiere-per-la-regia/ (translated)