The Republic P-44 Rocket was a progressive development of the P-43 Lancer. It can be regarded as yet another step along the road which led ultimately to the P-47 Thunderbolt.
As described earlier, the AP-4 project was a progressive adaptation of the Seversky P-35 fighter powered by a turbo-supercharged Pratt and Whitney R-1830-35 radial engine. The supercharger was mounted underneath the rear fuselage and was fed by an air intake mounted beneath the engine. On March 12, 1939, thirteen service test models of the AP-4 were ordered by the Army under the designation YP-43. Although the weight of the YP-43 was excessive, the turbosupercharger gave the new aircraft a considerable advantage in both speed and operational ceiling over the earlier P-35.
However, by 1939, the Lancer was already outdated by the rapid advances in air combat technology that had taken place in Europe. It suffered from poor maneuverability and climbing performance, and lacked such modern innovations as armor protection for the pilot and self-sealing fuel tanks. Consequently, the Army did not anticipate ordering any more P-43s beyond the initial service-test contract.
On September 13, 1939, the Army ordered eighty examples of the more advanced AP-4J from Republic under the designation P-44. The P-44/AP-4J was basically similar to the P-43 but was provided with the more-powerful Pratt and Whitney R-2180-1 radial engine of 1400 hp. Drawings of the projected P-44 show an aircraft which looked very much like the P-43 but with a somewhat longer nose and a somewhat heavier armament of six machine guns.
However, combat reports coming out of Europe in the spring of 1940 indicated that even the P-44 Rocket would not be up to the task, and Alexander Kartveli and his design team began to consider an even more advanced project known under the company designation of AP-10. A prototype of the AP-10 had been ordered by the USAAF in November 1939 under the designation XP-47. Since the USAAF regarded the XP-47 as showing greater promise, they cancelled all work on the P-44 project on September 13, 1940, before any P-44 prototype could be completed. 170 P-47Bs and 602 P-47Cs were ordered in their place.
Kartveli and his team then concentrated all their efforts on the P-47 project, which was to turn out to be a wise decision indeed. However, P-47 development promised to be protracted, the first production aircraft not scheduled to roll off the production lines until late 1942. The Army felt that Republic's Farmingdale production lines needed to be kept busy in the interim. Consequently, the P-43 was ordered into production as a stop-gap measure.