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The Martin Baltimore I was a light bomber developed from the earlier Maryland by increasing the width and depth of the fuselage and installing more powerful engines and more guns.
Compared to the Maryland the Baltimore had a significantly wider fuselage, which in theory allowed all four crew members to move around, but in practise the aircraft was still narrow enough to make it very difficult for the crew to change positions. The Baltimore had the same wingspan as the Maryland, but was slightly longer.
Power was provided by two 1,660hp Wright GR-2600-19 Cyclone engines, which required larger engine nacelles than on the Maryland. The extra power more than made up for the increased weight of the Baltimore, which when introduced was a “hot” aircraft, with high performance in the air but tricky ground handling.
The Baltimore I was delivered with ten guns installed – four fixed forward firing guns in the wings, four fixed backwards firing “scare guns” and one gun in each of the ventral and dorsal positions, on manual mountings. These were all rifle calibre guns (0.303in in the UK, 0.30in in the US), although sources disagree on exactly which calibre was used on the Baltimore I. What is agreed is that the ventral and dorsal guns were originally American 0.30in guns in an awkward installation, which were soon replaced by British guns. A number of the Baltimore Is had the single dorsal gun replaced by a twin mounting.
The first Model 187B Baltimore made its maiden flight on 14 June 1941. Of the original British order for 400 aircraft, 50 were delivered as Baltimore Is and 100 as Baltimore IIs. Four squadrons would use the Baltimore I (Nos.55, 69, 203 and 223), all of which was also operate the Mk.II.