B Mk I Series 1
The first production Halifaxes were powered by the 1,075 hp Merlin X engine. They carried 1,552 gallons of fuel in six tanks located between the two engines on each wing, with the capacity to add fuselage tanks to bring the maximum capacity up to 2,242 gallons, although this would massively reduce the bomb load.
The normal bomb load was 13,000 lbs. This remained constant throughout the Halifax's career. The bombs were split between one main bomb bay in the fuselage, capable of carrying 10,000 lbs of bombs and six mini bomb bays between the inner engine and the fuselage, each capable of carrying 500 lbs of bombs. This would later make the Halifax suitable for anti-submarine warfare, with depth charges in the wings and fuel in the main bomb bay.
Defensive firepower was provided by two Boulton Paul turrets – a two gun Type C in the nose and a four gun Type E in the rear, both armed with .303in Browning machine guns. In addition the Mk I could carry two Vickers gas-operated guns on each side, in staggered positions, which allowed both sets of guns to be operated at the same time. However, these guns were fired through open windows, which forced the gunners to wear heated flight suits if they were not to freeze!
Fifty Mk I Series Is were produced. The first production aircraft flew on 11 October 1940. No. 35 Squadron was the first unit to carry out a raid with the Halifax – a raid on Le Havre that saw one aircraft shot down by friendly fire while returning to base.
B Mk I Series 2
The main change made to the Mk I Series 2 was increased strength, which allowed an increase in total weight from 55,000 lbs on the Series 1 to 60,000 lbs. Twenty five Series 2 aircraft were produced.
B Mk I Series 3
Nine Series 3 aircraft were made, with relatively minor changes from Series 2. Fuel capacity was increased to 1,882 gallons by adding a fifth 122 gallon tank beyond the outer engine. Fuel jettison pipes were also added, to allow the pilot to jettison fuel before making an emergency landing. Series 3 aircraft used an engine cowling designed for the Merlin XX engines used in the Mk II. In total 84 Mk I Halifaxes were produced.