All the British four-engined heavies (and the Manchester before the Lancaster) were designed from the outset to be fitted with ventral turrets, but more often than not the turrets were awkward to use and were removed. In the Stirling, the FN.25 turret was prone to vibration and got filled with dirt when operating from grass. The Preston Green turret was a cupola mounting like what was originally fitted to the bombers, but it was unpowered and manually operated.