Early marks of the Whitley featured bomb bay doors, fitted on both the fuselage and wing bays, that were held shut by bungee cords; during bombing operations, these were opened by the weight of the bombs as they fell on them and closed again by the bungee cord. The short and unpredictable delay for the doors to open led to highly inaccurate bombing. The Mk.III introduced hydraulically actuated doors which greatly improved bombing accuracy. To aim bombs, the bomb aimer opened a hatch in the nose of the aircraft, which extended the bomb sight out of the fuselage but the Mk IV replaced this hatch with a slightly extended transparent plexiglas panel, improving crew comfort.