Originally intended as a high-speed replacement for the Il-4, it was recast as a very long-range bomber with fuel-conserving diesel engines before production of the single prototype began in December 1942.

Flight testing showed controllability issues when landing at high weights and the engines proved to be hard to start at low temperatures and were slow to respond to throttle movements.

Outwardly similar to the Il-4, the Il-6 was faster and had a longer and slimmer fuselage fitted with a completely new, highly tapered wing with an aspect ratio of 8.

Defensive armament was greatly improved with five 20 mm (0.79 in) cannon fitted on flexible mountings in the nose, dorsal turret, two waist positions and a ventral blister forward of the tailplane.

Throughout the flight tests the engines performed satisfactorily in the air, but were found to be very difficult to start in low ambient temperatures and had slow response to throttle movements.