The road to the American classic war-winning Republic P-47 "Thunderbolt" of World War 2 went through various iterations under the leadership of engineer Alexander Kartveli. In 1937 the United States Army Air Corps (USAAC) took into service the Seversky P-35 which was authored by Kartveli under the Seversky brand label (Seversky was reorganized in 1939 to become "Republic"). The P-35 was notable for it becoming the first American-made single-seat, single-engine monoplane fighter to feature all-metal construction, a fully-enclosed cockpit, and a completely retractable undercarriage. From this form the P-43 "Lancer" was eventually developed and arrived in 1941 to be used by the air services of the United States, China and Australia with production reaching 272 units before the end.
Even before the P-43 came to fruition, there was a stop at another Republic fighter offering - the P-44 "Rocket". The fighter was developed to a new U. S. Army requirement for an interceptor / pursuit type capable of speeds in the upper 300mph, lower 420mph range while flying under 20,000 feet of altitude. Republic beat out other submissions with their "AP-4J" which promised to fulfill the required specs.
Design work was, again, headed by Kartveli and drive power would stem from a single Pratt & Whitney R-2180-1 "Twin Hornet" engine of 1,400 horsepower fitted to the nose. A monoplane wing arrangement was, of course, in play and showcased rounded wingtips. The fuselage was well-contoured with the radial piston engine air-cooled and shrouded by a very tight cowling. A traditional single-finned tail unit was positioned to the rear in the usual way. The "tail-dragger" undercarriage was completely retractable. The cockpit, with its raised fuselage spine, was heavily framed and seated a single operator at midships. Armament was to be wholly machine gun-based: a mix of 2 x 0.50 caliber heavy machine guns paired with 4 x 0.30 caliber medium machine guns (a common arrangement of pre-war/early-war American fighters).
The AP-4J was estimated with a maximum speed of around 385 miles per hour and the Army thought enough of the Republic initiative to award a contract to the company for 80 aircraft on September 13th, 1939. Rather notable was the lack of any working, flyable prototypes to ensure a sound design. By this time, the war in Europe had just begun (September 1st) and reports from the front became critical to observers stateside and it was quickly realized that the modern mounts of Europe outclassed those being offered by the Americans.
The AP-4J was evolved into the AP-4L which was to install the Pratt & Whitney R-2800-7 series air-cooled radial of 2,000 horsepower. Additional internal fuel stores would be provided to help increase range. Cockpit armoring was now an essential quality of fighting warplanes as were self-sealing fuel tanks so these too found their way into the revised P-44 design - which was now ordered by the Army as the P-44-2 on July 19th, 1940. The initial contract called for 225 fighters to the newer standard and this was increased to 827 on September 9th of that same year. Even despite the added weight, Republic engineers were optimistically hopeful that their new fighter would hit the 422mph maximum speed envelope.
However, as soon as it arrived on the drawing boards, the P-44 Rocket was made more or less obsolete by events half-a-world-away. Fortunately for Republic it had also been hard at work at developing another fighter in the "AP-10" which also caught the Army's eye back in November of 1939 - this aircraft becoming the prototype XP-47 before being finalized in service as the classic P-47 Thunderbolt. With the XP-47 proving itself the more promising venture, the P-44 project was ended on September 13th, 1940 with no physical prototype to show for the years-long effort - such was the military aircraft design business. To keep Republic production lines open until P-47 manufacture could be brought up to speed, the P-44 contract was simply converted by authorities to purchase more P-43 Lancer fighters for the U.S. Army and its ally in China.