The Republic P-47 first took shape as the Seversky P-35 before that company became Republic. The design was evolved through the P-43 Lancer before the requirements led to World War II's heaviest fighter, the P-47. The P-47 was designed with the typical streamlined cockpit of the day before rearward visibility in aerial combat became a requirement. Later P-47s were redesigned with the rear upper deck cut down to facilitate a bubble canopy atop the fuselage which provided excellent all-round visibility. This new design was introduced part-way through P-47D production, which for whatever reason didn't warrant a new model designation. P-47s were among the first fighters to accompany the USAAF over to Europe as the United States entered the war. While many fighter wings were quick to transition to the P-51 Mustang as soon as it was available, the 56th Fighter Group was one of the only organizations to refuse the Mustang and see the war through in their beloved Thunderbolts.