Fortunately, Republic had been paying attention to developments with the P-51 and were hard at work on a long range Thunderbolt as early as November of 1943. The same P-47C-5-RE that had been used as development mule for the Pratt Whitney R-2800 C series was fitted with 27 inch long extension inserts in each wing. This was done largely to test the handling of the aircraft. No provision was made to carry any additional fuel in the wings. These tests revealed that the roll rate had suffered and the wings were clipped at the tips and a squared off cap was fitted.
This is the only existing photo of the "unofficial" XP-47M test mule. After being used as a testbed for the Pratt Whitney R-2800 C series engine, it was fitted with 27 inch long wing extensions (at the wing root), and used to test the flight characteristics of the modified wing. When the request for a long range Thunderbolt came along, this fighter was wheeled out, dusted off and presented to the Air Material Command. The photo shows the fighter prior to receiving the modified wing.
In May of 1944, an Expenditure Order was issued and $101,000 was allocated to Republic to develop a "wet" wing to be installed on one of the three YP-47M prototypes. Within 30 days Republic was able to present a test report based upon their earlier testing, along with a full set of drawings as a proposal to the Air Material Command. The new drawings illustrated the new wing design. The inserts were now just 18 inches in length, and contained an integral fuel tank for 100 gallons of fuel. In the contract, the #3 YP-47M was specified as the test aircraft (S/N 42-27387). Twenty hours of flight time were expected after the new wing had been installed. In July, the fighter was officially designated as the XP-47N and the unofficial test mule was re-designated as a P-47C-5-RE once again. The final disposition of the test mule is unknown. It did, however, retain its new wings and the more powerful C series engine for as long as the aircraft appeared on the company inventory.
The modifications to the existing YP-47M were considerable. Aside from simply installing the wing inserts and fuel tanks, the flaps were required to be redesigned, and the ailerons had to be modified to fit with the new squared-off wing tips. Due to spacing the wings out from the wing root, the landing gear track increased by more than 3 feet. The overall wing span had increased to just over 42 ft 6 inches. The empty weight of the fighter had gone up by nearly half a ton to 12,950 lbs.
This wonderful view of the XP-47N shows its new wing planform to great effect. The squared-off wing tips makes P-47N indentification easy. Note that the Prototype fighter does not yet have the dorsal fillet installed.
The XP-47N took to the air for the first time on July 22, 1944. Test comparisons were made with a P-47D-30-RE throughout the early portion of the evaluation period. Much to everyone's surprise, the XP-47N, with its greater wingspan and higher weight actually proved to have better roll performance than the D model. At 250 mph TAS, the N attained a maximum roll rate just over 100 degrees/second. The P-47D-30-RE could manage but 85 degrees/second at the same speed. At higher speeds, the N widened the gap further. In mock combat with a P-47D-25-RE, the new fighter proved to be notably superior in every category of performance. In short, the XP-47 waxed the venerable D model regardless of who was piloting the older fighter. The new wing was part of this newfound dogfighting ability, however, the more powerful C series engine played a role too. The additional horsepower allowed the N to retain its energy better than the older Thunderbolt.