The FN25 was a retractable gun turret designed to be lowered under the Vickers Wellington bomber to provide protection from fighters attacking from below (the ventral position). It had a very short lifespan, being developed to replace the Vickers turret used in the 181 Wellington Mk 1s, and was used in the Mk 1A. It was used in the 187 Mk 1As produced early in the war, but abandoned for the much more numerous Mk IC.
The FN25 was very similar to the FN17 used in the Armstrong Whitworth Whitley. When lowered into position, it resembled a dustbin hanging below the aircraft. The turret required two sets of hydraulics, one to lower the turret into place and another to rotate the turret. In normal flight the turret would be retracted in the body of the aircraft. To use the turret, the gunner would first lower it into place, and then lower themselves into the turret, before finally lowering a footwell.
The FN25 carried two 0.303in Browning Mk II machine guns with 500 rounds per gun. The turret could rotate through 360 degrees, although the guns could only be lowered to 50 degrees below horizontal. Visibility was poor – the turret was almost entirely metal, with a narrow window running from top to bottom along the front of the turret. When the turret reached squadron service in 1939-40 it was a total failure. It slowed the Wellington down by 10 mph just when maximum speed was essential. The poor visibility made it impossible to track an enemy fighter. Several had to be jettisoned when they jammed in the down position. Many of the FN25 turrets were removed, saving their 380 lbs of weight. However, the ventral turret was not a total failure. The hole in the fuselage was found to be ideal for other uses, including dropping paratroopers. More significantly, the hole left by the removal of the FN25 was later used by the Leigh Light, an anti-submarine weapon, and the surviving FN25 turrets provided an excellent framework for carrying the light.