Pratt & Whitney followed the Wasp Junior A with more powerful models in the "A series". These had higher compression ratios, greater RPM limits, and more effective supercharging, and they led to the "B series". The first B series model was the Wasp Junior TB, which could maintain at sea level and could reach for takeoff. The TB was tuned for best performance at sea level; it was soon joined by the Wasp Junior SB, which was tuned for best performance at altitude and could sustain at altitudes up to , with available for takeoff. A still later model, the Wasp Junior T1B2, had improved performance at low level, being able to sustain up to while still matching the SB's power at high altitudes. The SB and T1B2, and later versions of these with similar performance, were the most popular Wasp Junior models. One later development of the T1B2, the Wasp Junior B4, was especially designed for vertical mounting in helicopters.