Well, they are not kill marks. The aircraft was stationed at Edwards AFB in California for most if not all of its career. Each Symbol denotes a peticular test mission. In Example
The two B-52 silhouettes at center represent Wake Vortex tests and one represents an F-16 Target Test. Ten marks represent flights by the F-15 Spin Research Vehicles, although the three vehicles were launched twenty-six times. The first F-15 SRV flight was launched on November 29, 1977 and the last was launched on July 15, 1981.
Thirteen marks represent the Shuttle Reusable Booster/Drop Test Vehicle (SRB/DTV). Two horizontal silhouettes at upper right indicate captive carry flights flown on June 10, 1977. Below those silhouettes are two more representing the first series of six drops of the SRB/DTV, which began on June 15, 1977 and ended on September 12, 1978.
The nine SRB/DTV marks to the left represent a second series of drops of the SRB/DTV. Three large stabilizing fins were added to the SRB/DTV. The SRB/DTV silhouettes for the second series of drops show the fins. The first drop was scheduled for February 23, 1983, but the rear hooks on the X-15 pylon failed as the NB-52B was taxiing. The horizontal silhouette with the hook at the front represents that incident. The first of eight drops in the second series was flown on September 16, 1983. The last was flown on March 20, 1985. The parachutes separated from the SRB/DTV on one flight, which is represented by a silhouette with a ground line and Joshua Trees on each side.
Go here to learn more of what all the symbols represent. I used the info off the website for the above pictures.
Goleta Air and Space Museum: NASA's NB-52B Walkaround