Vought XSO2U

During the late 1930s the U.S. Navy developed a set of specifications for a new scout-airplane capable of operating from cruisers in the reconnaissance and gunnery spotting roles. This new machine was intended to replace the Curtiss SOC Seagull biplane. This aircraft was required to have folding wings, a superior range and speed to that of the Seagull, it had to be powered by the Fairchild Ranger V-770 inline engine.
Both Vought-Sikorsky and Curtiss-Wright submitted their designs. Vought's design, named internally as Model 403, was identical to the Vought Kingfisher, which was back then in development, intended to replace the Seagull on Battleships, but had its wings moved higher on the fuselage, when compared to the Kingfisher, and it was different in the attachment method used by its single floater landing gear. Additionally, the radial engine of the Kingfisher was replaced by the Ranger V-770 inline engine which yielded 450 hp of power.
The XSO2U was capable of operating both on floats as a seaplane, or with a conventional fixed undercarriage, as a landplane. It was entirely made out of metal with the exception of its control surfaces which were fabric-covered. The wings could be folded to rear for storage in a way similar to those of the Grumman TBF Avenger torpedo bomber.
The aircraft had diving bombing capabilities and could be fitted with a single bomb or depth charge on hardpoints under each wing. It was armed with two M2 0.5 in (12,7 mm) Browning machine guns, one forward firing through the propeller using synchroniser gear and the other in a flexible position in the observer's cockpit for defensive purposes.
The XSO2U flew for the first time in July 1939 in the landplane configuration and it wasn't until December that year that it flew with floaters. Flight testing showed that the aircraft lacked directional stability, but the addition of a larger ventral fin, connecting the rear of the float to the tail, helped to solve the problem.
It also suffered from engine problems as the Ranger engine wasn't known for its reliability. It was particularly prone to overheat and a solution for that problem could never be found. The prototype's original engine, V-770-4 was replaced by a XV-770-6 which featured a repositioned oil cooler, but the problem persisted.
In spite of the engine's overheating problems, it was considered superior to the competing Curtiss XSO3C-1 Seamew. However, Vought's manufacturing capacity was already taken by the Kingfisher and the F4U Corsair fighter (the first version of which was already into production in 1942), so the XSO3C-1 was considered as the winner of the contract and entered, therefore, into production.
After the competition, the XSO2U was used as a general utility aircraft and hack by the US Navy, before being given to the Ranger Engine Corporation in July 1942 for testing the V-770 engine. This testing was intended to help debugging for the Bell XP-77 light fighter and the Edo XOSE floatplane. The engine kept on being problematic and, after two years of testing, it was returned to the US Navy. As the airplane had no use for the Navy, it was therefore written off on 6th July 1944 and was subsequently scrapped.

Sources:
1. https://en.wikipedia.org/wiki/Vought_XSO2U
2. Mc Farland - United States Naval Aviation, 1919-1941
3. Signal Squadron - Aircraft In action 119 - Vought OS2U Kingfisher in Action