In the late 1940s, it had become clear to the Navy that the then-standard 20mm cannon armament was unlikely to be effective against jet bombers. It would take a lot of bullets to bring one down and the geometry of the attack meant there was not enough time to do that with deflection shooting. A classic tail-on attack, which might provide the requisite firing time, meant dueling with the bomber's tail gunner, not a good option.
The solution was thought to be a salvo of 2.75-inch folding-fin rockets. They weren't very accurate, but only one had to hit the bomber to have a high probability of bringing it down. Two interceptor programs were initiated to provide this capability, the bat-winged Douglas XF4D with externally carried rocket pods, and the sleek McDonnell XF3H with an internally mounted pair of launchers loaded with a total of 26 rockets.
As it turned out, the combination of fire control system and rockets was much less accurate than expected, with even one hit being unlikely. Fortunately, air-to-air guided missiles were in development that proved adequate to the task.
The illustrations are from a 1949 Navy Confidential Bulletin.