As an experiment to improve the rearward visibility, the USAAF fitted a standard P-47D-5-RE airframe (serial number 42-8702) with a bubble canopy taken from a Hawker Typhoon. In order to accommodate the bubble canopy, the Republic design team had to cut down the rear fuselage. This conversion was redesignated XP-47K, and was tested in July 1943. This modification was immediately proven to be feasible, and was promptly introduced on both the Farmingdale and Evansville production lines.
Ordinarily, the USAAF would have given such a radical modification as that which produced the bubble-canopy Thunderbolt a completely new variant letter (or perhaps even a new type number). However, the USAAF chose instead to designate it simply by giving it a new production block number in the D-series. Consequently, the first batches to feature this new bubble canopy were Farmingdale's P-47D-25-RE and Evansville's P-47D-26-RA.