I would say the idea for the kind of propulsion was used because of possibility of getting of the best forward visibility for the observer in the first cockpit. What is more the way the plane was powered doesn't seem to be the ducted fan. The four-blade prop was attached to the ring spinning around the fuselage just behind the wings. Here a couple of shots found via the net where the prop can be seen better. So no tunnel there for the prop.
the pic source: the net.
the pic source: the net.