The Douglas XB-42 Mixmaster had a maximum take of weight of 16,194kg (35,702lbs). (Note - the later A variant had a couple of underwing jets, which took the mtow above 20,000kg.) It was powered by two Allison V-1710 engines each independently connected to one of the contra-rotating propellers. In an emergency, the entire rear assemblage could be explosively jettisoned.

There is the Douglas XTB2D Skypirate with a maximum take-off weight of 15,767kg (34,760 lbs).
It had a single Pratt & Whitney XR-4360-8 supercharged 3000hp radial piston engine driving a pair of 14.3-foot diameter, contra-rotaing four-bladed propellers.
The first U.S. Navy Douglas XTB2D-1 Skypirate prototype (source)
Both these planes have contra-rotating propellers. For the largest plane with a single propeller, it may be the Linke-Hofmann R.II of 1917, with a gross weight of 12,000kg (26,455lbs) . It is said to have had the largest propeller ever flown, its four engines turning a single 6.90m (22 ft 7.5 in) diameter two-blade propeller at just 545rpm.
(source) looks normal until you notice the people
And... this one is a bit of a cheat, but also the Boeing JB-17G engine test-bed. Since the B-17G had an empty weight of around 17,000kg, it comfortably beats the other contenders.
In flight its main engines were shut down, and it was flown with a single engine.
