X-Plane 9.70+ Martin XB-51. The Martin XB51 was an American "tri-jet" ground attack aircraft. It was designed in 1945 and made its maiden flight in 1949. It was originally designed as a bomber by the United States Army Air Forces under specification V-8237-1 and was designated XA-45. The "A" ground attack classification was eliminated the next year, and the XB-51 designation was assigned instead. The requirement was for low-level bombing and close support. The XB-51 lost out in evaluation to the English Electric Canberra which entered service as the B-57. This a revised version of the Martin XB-51 tactical bomber by Norvell. By Vasco Ribeiro.