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.

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

The SB-RK was not only a further perfectioning of the unsuccessful SB-MMN, but it was projected for dive bombing too.
In 1939, TsAGI (The Central Istitute for Aero and Hydrodynamics) had developed a special bomb lock called PB-3, that allowed SB to throw bombs while diving.
First tests were performed by the crews of Major Zhdanov, Captain Kovalchuk and Senior Lieutenants Subbotin and Piskunov, on an SB-2-M-105, 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, an automatic command was actioned to recover the aircraft from dive immediately after pushing the bomb release button.
An overload limitator was developed. This limited the overload to a pre-setted value (usually 2,9 g) whatever the diving angle, speed, bar position were.

When compared to SB-MMN, SB-RK featured:
 

new engine frames and nacelles with improved aerodinamicity; new water coolers in tunnels inside the wing thickness, with inlets on the wing leading edge and outlet on the wing uppersurface; new lubrification system, with a water-oil cooler and a air-oil one on each engine; the air-oil had a circular intake on the wing leading edge; VISh-22E propellers; air brakes under the wings;

new wings outer panels, similar but not identical to those of MMN, and with slightly greater wingspan (18.5 m instead of 18.0 m);  new 330-l fuel tanks in each wing outer console  (instead of 205 l of MMN); the covering of the top fuselage between 4-th and 8-th frames was carried out from "balenit", the plywood pasted by VIAM glue  to wooden carrying parts of 5-th, 6-th and 7-th frames;

new nose with more space for crew and a glazed front; nose ShKAS was installed in a new spherical installation called NU-DB-3F for the navigator; dorsal ShKAS  was in dorsal installation TSS-1  the ventral ShKAS  with sight OP-2P was placed inside a fuselage turret MB-2 the seat of the pilot was moved on the left, while the instruments panel was moved on the right to allow the pilot to see through the windows of the navigator at landing and on a dive; there were duplicate flying controls in the navigator's cabin (as on MMN); there were an NKPB-3 sight (for bombing at night and from small altitude) and an OPB-1 sight, both in the navigator's cabin; there was a PBP-1 sight (for dive bombing) in the pilot's cabin; there were a diving gyro artificial horizon and an overload alarm on the pilot's instrument panel; new improved bomb racks were installed.

Factory test of two prototypes of SB-RK started in October 1940. The main defect was the overheat of water (95-100°C) and oil (110-115°C), and insufficient longitudinal stability.
The third prototype was completed in November 1940, and it was transferred to NII-VVS for state tests.
This aircraft had some improvements on the first two prototypes: the shutters at the outlet of water cooling tunnels was reduced to 5, the rudder depth was changed.
In December 1940, an order of NKAP (the People's Commissariat for Aviation Industry) renamed the aircraft as Ar-2, from the surname of its designer A.A. Archangelskij.
The state tests were completed in January 1941; the head engineer was M.I.Yefimov, while the test pilots were Major W.I.Zhdanov and Captain A.M.Hripov.
 

Amongst the performances measured by NII-VVS there were: max speed at 6600 m altitude, with a weight of 6600 kg, 475 kim/h climb to 5000 m: 7,1 minutes practical ceiling with normal flight weight 6600 kg (without external bombs): 10000 m; practical ceiling with two FAB-250 under the wings, 7100 kg: 9000 m; range of flight with two FAB-250 in internal bomb bay, at 390 km/h: 990 km; takeoff length at 7100 kg: 340 m.

The plane was able to fly with one engine only.
Many diving tests were made.
The second prototype SB-RK,designated N1/281, was tested in dive bombing with angles up to 75°.
When starting a dive from 4000 m altitude with a speed of 290 km/h, it lasted for 9 s (sufficient for aiming bombs), then, after dropping bombs, it automatically ended the dive at 550 km/h maoeuvering at 4,5 g.
Both the brakes, the bombing release mechanism and the automatic device to end the diving at bomb dropping functioned trouble-free.

The cooling and lubrificating system of the M-105 engines was a terribly weak point. The full power of the engine caused water overheating at 110°C when the ambiental temperature was only -10°C. The lubrification system overheated and destroyed the radiators 12 times during the tests.
The armament installation had some defects too, that caused vibrations, bad stability of machine guns and delays in firing.
Besides, the 7,62 mm ShKAS were not lethal enough against modern fighters; this defect was common to all the Soviet bombers of the time.
The report of NII-VVS said that Ar-2 was a great improvement over the SB, but its speed it was not comparable with better Soviet and foreign bombers of its age.
The handling of Ar-2 was good, even easier than SB.
The aircraft was good enough for operative use, should its main defects be eliminated.

In February 1941, a new prototype (N1/511) was delivered to NII-VVS; the engines M-105R were moved forward by 150 mm for balancing reasons; the VIT1T-22E propellers with 3.1 m diameter were adopted; the reduction rate of engine gears was 0.59 instead than 0.66.
Air brakes were thinner, the exhaust stacks were shaped to obtain some jet thrust, and the surface finish was improved.
Thank to these improvements, the aircraft reached a speed of 443 km/h at ground level, and of 512 km/h at the altitude of 5000 m.

The aircraft was pushed in production in late 1940, but the availability of the faster Pe-2 and the first flight of the new plane "103" "AM-37 (the prototype of  Tu-2) made clear that this was an interim solution. The "103" was able of more than 600 km/h, was strongly armed and it turned any other existing bomber into an obsolete one. However, Tui-2s got full operational state only in 1944.
So, 190 Ar-2 were produced in Zavod 22 when it turned the production to the Pe-2 in the first half of 1941.

The operative life of Ar-2 was very limited not only due to its limited production, but also because a lot of them were destroyed or captured on the ground or sent to attack the enemy without any escort in the first days of war. According to official sources, 95 Ar-2 were lost during 1941.
Some authors regret the stop of production of Ar-2 in favour of Pe-2 because, if it's true that Pe-2 was 20-40 km/h faster and had a greater range, it's also true that the Ar-2 had better bombing qualities and was able to accept rough landings by far better than the Pe-2, many of which were written off in this way when their fuselage broke.
 
 

Ar-2 prototype photo gallery

Some images of Ar-2 prototypes in 1940.