Once the Air Force of each country started paying their attention to night bombing, the bombing became more effective than ever. To fight against the night bombing, a series of night fighter were put into use. The Americans started their night fighter program in 1940.The Army use the P-70 Havoc and Beau fighter to fight with the Japanese until the first 45 P-61 Black Widows were in active service in October,1943.

With the birth of the more powerful Airborne Intercept radar, the SCR-720C, the P-61s of the Army began their upgrade to install this latest radar. The new type of Black Widow with the SCR-720C were designated as P-61B.The type B length the nose section and reintroduced the dorsal turret on the later production