Watch the video. After getting airborne, it appears Mr. Hughes actually levels the aircraft to keep it from climbing further. The aircraft does not appear to be anywhere near a stall. Hughes aircraft designs before the H4 Hercules and after the H4 all flew. Some quite well, some broke records. Anecdotal information indicates the tail structure of the H4 was weak and would require extensive reinforcement before any subsequent flights, however. Given the state of aerodynamic knowledge in the 1940s, it would be extremely rare for a design to progress that far, and then be unable to sustain level flight out of ground effect.