The SEV-1XP single-seat adaptation of the SEV-2XP initially retained the R-1670 engine. By now featuring a semi-retractable undercarriage, the prototype was reengined with an ungeared nine-cylinder Wright R-1820 Cyclone. This engine failed to produce its rated power, however, the predicted 483km/h maximum speed proving unobtainable in consequence. The USAAC Materiel Division's decision to defer fighter choice pending further evaluation to be held in March 1936 enabled the SEV-1XP to be re-engined once more, this time with an 850hp 14-cylinder Pratt & Whitney R-1830-9 Twin Wasp. This was succeeded, in turn, by a geared R-1820-G5 offering 950hp for take-off, but the Twin Wasp was subsequently re-installed. With this last engine and an armament of one 7.62mm and one 12.7mm gun, the SEV-1XP was selected on 16 June 1936, approval being given for production of 77 fighters against a contract confirmed early in 1937, with the designation P-35.