It was a YB49, the type also featured in the HG Wells film "Shape of things to come".
I think that 11 were built, but only one was flown. The lead Test Pilot for the early part of the programme was Maj (later Gen.) Bob Cardenas, who is still around. The aircraft was later lost under the control of another TP (sorry I don't have the names to hand) resulting in the loss of a complete test crew on 15 June 1948. This led to (but wasn't the sole reason for) abandonment of the project and all the surviving prototypes were broken up. It's generally accepted that the work done on that led to the B2 from the same stable some years later - once fly by wire technology had made control of such a machine much more practical.
The following is a modified extract from some (as yet) unpublished work by myself - although the actual flight test report was courtesy of General Cardenas:-
G
The following account is by a USAF Test Pilot working in 1948 upon evaluation of the YB-49 aircraft, and describes a pitching loss of control in this aircraft. The use of the word “tumble” is that selected by the Test Pilot at the time.
“23 February YB-49 #368 one landing local Muroc-------- 0:35 mins.
Recommended no intentional stalls due to the fact that during the final phase of the stall entry maneuver it lurched over backwards into a tumble. Had to use asymmetric power to recover. Submitted a full report and thankful that the throttles were hanging down from the ceiling rather than in a normal position since G forces had my arms locked upwards and my rear off the seat. Flight test engineers told me later that I had encountered inertial coupling”
“the results of my one Stall Test during which the aircraft had assumed a very high angle of attack without a stall warning and then pitched over backwards…. The rotation was severe and made it difficult to keep my hands and feet on the controls. The engineers called it a lateral roll but I was experiencing a tumble! I was lucky that the designers had put two throttles hanging down from the upper surfaces, each connected to four engines.I applied full power with the left throttle and resolved the "tumble" with asymmetric power and elevon control.”
The aircraft was subsequently lost on 15 June 1948 whilst under the control of another Test Pilot and was destroyed killing all on board. Available reports indicate that the aircraft lost control in pitch at about 40,000ft, with the wingtips detaching from the airframe at a high altitude under loading which exceeded 4.8g[ ]. The aircraft descended almost vertically, impacting inverted, whilst the wingtips were found several miles away.
