The first prototype XP-84 was transferred to Muroc Army Air Field (present-day Edwards Air Force Base) where it flew for the first time on February 28, 1946, with Major William A. Lien at the controls. It was joined by the second prototype in August, both aircraft flying with J35-GE-7 engines producing 3,745 pound-force (16.66 kN) of thrust. The fifteen YP-84As delivered to Patterson Field (present-day Wright-Patterson Air Force Base) for service tests differed from XP-84s in having an upgraded J35-A-15 engine, carrying six 0.50 inch (12.7 mm) Browning M2 machine guns (four in the nose and one in each wing root), and having the provision for wingtip fuel tanks holding 226 US gallons (870 L) each. Due to delays with delivery of jet engines and production of the XP-84A, the Thunderjet had undergone only limited flight testing by the time production P-84Bs began to roll out of the factory in 1947. In particular, the impact of wingtip tanks on aircraft handling was not thoroughly studied, which proved problematic later.