The crew were returning from a ''low level'' mission in the UK to RAF Laarbruch in Germany when they were flying through Dutch airspace over the North Sea.
Without warning the UFO - as big as a C130 Hercules plane - appeared and they had ''never seen anything like it''.
In written answers about the incident the pilot said: ''I did not lock the UFO on radar. My navigator and I were so surprised we did not think to do so.
''Indeed, for the majority of the sighting, the UFO was out to one side of my aircraft which would have required me to manoeuvre the aircraft to place within the radar field of view.
''Of more interest, despite repeated radio calls to Dutch Military Radar, the controller insisted that he could not see the UFO.