From Baughers site:
"As early as 1942, the Army had concluded that the Liberator would have better aerodynamic stability if it had a single fin and rudder. However, the Liberator was destined to go through almost its entire career with its original twin fin-and-rudder assembly.
In early 1943, Ford/Willow Run decided to test this assumption of better stability with a single fin and rudder. They modified a B-24D airframe to accommodate a single vertical tail unit taken from a Douglas B-23 Dragon. This aircraft was initially known as B-24ST (where the ST stood for *Single Tail*), and made its first flight on March 6, 1943. Following a change to a C-54 tailplane and a new rudder, the new fuselage was attached to another, later production B-24D airframe (B-24D-40-CO 42-40234). At the same time, it was fitted with more powerful R-1830-75 engines, each developing 1350 hp for takeoff. This airframe was also fitted with the power-operated nose turret that had been installed on later Liberators, while retaining the Consolidated tail turret.
This highly modified aircraft, designated XB-24K, flew on September 9, 1943. Tests revealed that the new tail configuration did indeed greatly improve the stability and handling of the Liberator. An additional benefit was an improvement in the field of fire for the tail gun. As a result of its additional engine power, the XB-24K was 11 mph faster than previous Liberators and had a much improved climb rate.
The results were so encouraging that in April 1944 the Army recommended that all future Liberators be manufactured with single tails. It was planned that the single-tailed Liberator would first appear on the production line with the B-24N version, but the approaching end of the war led to the cancellation of the B-24N contract after only 8 examples had been built. However, the single-tail configuration was later adopted for the PB4Y-2 Privateer. "
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