The BLU-1/B and BLU-1B/B 750 pound fire bombs are designed for external carriage on high performance aircraft with forced ejection release systems. They are constructed of aluminum with reinforced areas for sway bracing and aircraft forced ejection.
Unlike earlier types of fire bombs, the BLU-1/B utilizes electrically-armed impact fuzes which require a cable assembly to utilize power generated by a thermal battery in the initiator. End caps are used in the BLU-1/B instead of nose caps and tail cones and there are significant overall size and weight differences. In addition, the fuze system has an arming delay of .3 to 1.1 seconds.
The fire bomb BLU-1B/B differs from the BLU-1/B in that it has a beam assembly (hard back) which is extruded rather than cast, and an initiator adapter which is designed to prevent napalm from leaking into the initiator well. In addition, it has newly designed suspension lugs which allow sufficient clearance for the initiator when the bomb is loaded on an aircraft. Otherwise, the two bombs are identical and are packaged, stored, and assembled in the same manner. US Bomb, 750 lb Fire, BLU-1/B, BLU-1B/B (bulletpicker.com)