▁pro tot ypes ▁was ▁issued ▁on ▁ 1 ▁October ▁ 1 9 4 0 ▁with ▁the ▁aircraft ▁designated ▁the ▁XP - 5 3 . ▁ ▁Within ▁two ▁months ▁the ▁Army ▁Air ▁Corps ▁modified ▁the ▁contract ▁to ▁require ▁the ▁second ▁prototype ▁be ▁completed ▁with ▁a ▁Roll s - Ro y ce ▁Mer lin ▁engine ▁in ▁place ▁of ▁the ▁XIV - 1 4 3 0 . ▁That ▁aircraft ▁was ▁re - design ated ▁XP - 6 0 . ▁The ▁air frame ▁design ▁for ▁the ▁XP - 6 0 ▁was ▁modified ▁for ▁the ▁different ▁engine , ▁and ▁the ▁main ▁landing ▁g ear ▁was ▁changed ▁from ▁the ▁rear ward - ret ract ing ▁P - 4 0 ▁design ▁to ▁a ▁new ▁in ward ▁ret ract ing ▁version , ▁which ▁allowed ▁a ▁wider ▁wheel base ▁and ▁a ▁smooth ▁wing ▁surface ▁when ▁the ▁g ear ▁was ▁ret ract ed . ▁This ▁aircraft ▁first ▁fle w ▁on ▁ 1 8 ▁September ▁ 1 9 4 1 ▁with ▁a ▁British - built ▁Mer lin ▁ 2 8 ▁engine . ▁The ▁XP - 5 3 ▁prototype ▁was ▁then ▁converted ▁into ▁a ▁static ▁test ▁air frame ▁for ▁the ▁XP - 6 0 . ▁ ▁Consider ing ▁delivery ▁del ays ▁of ▁quantities ▁of ▁the ▁Pack ard - built ▁Mer lin ▁engines ▁due ▁to ▁its ▁use ▁in ▁other ▁f igh ters , ▁the ▁use ▁of ▁a ▁tur bo - super charg ed ▁All ison ▁V - 1 7 1 0 - 7 5 ▁engine ▁was ▁considered ▁in ▁its ▁place . ▁Con sequently , ▁on ▁