The first prototype, the Me 264 V1, bearing the "Stammkennzeichen" factory code of RE+EN, was flown on 23 December 1942. It was powered at first by four Jumo 211J inline engines of 990 kW (1,340 hp) each. In late 1943, these were changed to the BMW 801G radials which delivered 1,290 kW (1,750 hp). Trials showed numerous minor faults and handling was found to be difficult. One of the drawbacks was the very high wing loading of the Me 264 in fully loaded conditions at some . Comparable aircraft, such as the Boeing B-29 Superfortress with a wing loading of , the redesigned He 277 at and the Ju 390 at had lower wing loadings. The relatively high wing loading caused poor climb performance, loss of manoeuvrability, stability and high take-off and landing speeds. The first prototype was not fitted with weapons or armour but the following two prototypes, the Me 264 V2 and V3 had armour for the engines, crew and gun positions. The Me 264 V2 was built without defensive armament and vital equipment and the Me 264 V3 was to be armed and have the same armour.