In the beginning of 1940, the Archangelskiy construction bureau in Zavod 22 aircraft factory worked hard on a perfectioning of the Tupolev SB bomber.
Two steps of development were taken in consideration:
an aerodynamic improvement of the plane to achiece a top speed of 500 km/h at 6500 m with M-105 or M-106 engines;
a full redesign of the plane to reach a top speed of 600 km/h.
Dive bombing tests
In 1939, TsAGI (The Central Istitute for Aero and Hydrodynamics) developed a special bomb lock called PB-3, that allowed SB to throw bombs while diving.
Tests were performed by the crews of Major Zhdanov, Captain Kovalchuk and Senior Lieutenants Subbotin and Piskunov, and show the good functioning of this device.
During trials of dive bombing from at an angle of 80°", crews confidently stacked bombs in a circle with a radius of 57 m at height of dump of bombs of 2000 m and in a circle with a radius of 126 m at height of dump of bombs of 4000 m.
For safe recovering after dive, a overload limitator was developed. This acted on elevators, limiting the overload to a pre-setted value (usually 2,9 g) whatever the diving angle, speed, bar position were.
On July, 15th, 1940 army tests of five serial bombers SB with 2M-105 and airscrews VISh-22E in diving variant began. The bomb lock PB-3 was installed. Simultaneously, also bombers with PB-3 and propellers VISh-23 were tested, also in diving variant.
SB-RK, later Ar-2