The designation KB-29P was assigned to 116 B-29s that were converted to boom refuelling tankers by Boeing in 1950-51.
The hose refuelling system used by the KB-29M turned out to be extremely cumbersome and difficult to use in service. The time needed for tanker and receiver to make contact was usually quite long, the rate of fuel transfer was slow, and the aerodynamic drag imposed by the hoses limited the airspeed. Boeing went to work on the problem and came up with the flying boom technique which is still in use today. Basically, it replaced the tanker hose and its system of lines, winches, and reels with an aerodynamically-controlled swivelling and telescoping arm that was steered by an operator situated in an observation bubble that replaced the former tail turret. Aerodynamic control of the boom was managed by a set of rudders and elevators attached to the boom which controlled the azimuth and elevation. Extension and retraction of the nozzle was done hydraulically. The receiver aircraft had a receptacle on top of the fuselage to receive the nozzle, and during the refuelling maneuver, the operator would "fly" the boom nozzle into the receptacle of the receiving aircraft. Once contact was made, the transfer of fuel could begin. When not in use, the flying boom was latched onto a cradle which extended behind the tail of the tanker aircraft.
The boom system had the advantage in that relatively few modifications were needed in the receiving aircraft. However, the pilot of the receiving aircraft did have to keep the nose of his plane within a certain envelope behind and below the tanker in order to achieve contact with the end of the boom. The pilot of the receiving aircraft was assisted in his task by a series of indicator lights along the belly of the tanker aircraft which directed the pilot to move right or left, up or down, forward or aft.
In order to meet the demand for boom tankers, the remainder of the Renton plant (part of which had been opened for C-97A production) was reopened, and 116 B-29s were converted to KB-29P configuration in 1950-51. The first of these was was delivered to SAC in March of 1950.
Although the boom system was primarily used for the refuelling of large aircraft such as bombers, it could be used to refuel fighters as well. However, the probe-and-drogue system was found to be more useful than the boom system in the refuelling of smaller aircraft such as fighters. In this system, the tanker aircraft trailed a hose behind it, and a drogue at the free end of the hose was engaged by a probe on the nose of the fighter. A few KB-29P boom tankers were adapted to refuelling fighters by this technique in which a short length of hose and a drogue was installed at the end of the boom.