Six American fighter aircraft today, starting with the Republic P-47 Thunderbolt, one of the most famous fighter aircraft of the Second World War.
It had four precursors, starting with the Seversky P-35, the first fighter produced by the Seversky company. The Seversky XP-41 was a project that never entered production, being replaced by the Republic P-43 Lancer, an undistinguished fighter that saw limited service in 1940 (In 1939 Seversky was renamed Republic). The Republic XP-44 Rocket was the final stepping stone on the road to the P-47 Thunderbolt. It never progressed beyond the prototype stage and was cancelled in favour of the Thunderbolt.
Finally, the Republic XP-72 was a development of the P-47 Thunderbolt that did not enter production.