|Sikorsky S-25 G2,3,4|
|Designer||Igor I. Sikorsky|
|First flight||Winter 1915-16|
|Primary users|| Russia|
|Number built||8 G2, 5 G3, 4 G4|
|Developed from||Sikorsky S-22|
|Wingspan||30.8 m (101 ft 2 in) |
|Armament||G3: 6 MGs|
450–680 kg (1,000–1,500 lb)  of bombs[note 1]}
|Max Speed||G3: 126 km/h (78 mph) [note 2]|
|Climb||G3: 6,500 ft (2,000 m) in 17:00 [note 3]|
|Ceiling||G3: 11,500 ft (3,500 m) [note 4]|
|Endurance||G3: 5:00 [note 5]|
In winter of 1915-1916 the first Sikorsky S-25 Series G-2 Il'ya Muromets four-engine bomber was completed. Eight were built altogether. It was also known as the Kievskiy.
One gunner could assume a position atop the fuselage with his head above the top wing, with machine guns aiming fore and aft. Two other gunners were just aft of the wings using side windows in the fuselage, and a fourth was dragged to the tail on a small railed cart. This was likely the first tail-gunner position in any aircraft.
Five S-25 Type G3s were completed from early 1916 to 1917 followed by three during the Revolution, followed by roughly four S-25 Type G4 in 1917. The G-3s featured 220hp Renault engines plus 150hp R-BVZ-6 engines, twin fins, and armament of six machine guns.
S-25 Type G4s were built starting in July 1917. Perhaps only four were built before production was disrupted by the Revolution. 
For more information, see Wikipedia:Sikorsky Ilya Muromets.
- Durkota, p.323.
- Lamberton, pp.228-229.