The Beaufort came from Bristol's submission to meet Air Ministry Specifications M.I5/35 and G.24/35 for respectively a land-based twin-engined torpedo-bomber and a general reconnaissance aircraft. With a production order following under Specification 10/36, the Bristol Type 152 was given the name Beaufort after the Duke of Beaufort, whose ancestral home was nearby in Gloucestershire.
The competing torpedo bomber entry from Blackburn was also ordered as the Blackburn Botha. In an unprecedented step both designs were ordered straight off the drawing board, an indication of how urgently the RAF needed a new torpedo bomber. Three hundred and twenty Beauforts were ordered. Initially, because of their commitment to the Blenheim, Bristol were to build 78 at their Filton factory, with the other 242 being built by Blackburn. These allocations would be changed later.
Although the design looked similar in many ways to the Blenheim, it was somewhat larger, with an 18 in (46 cm) increase in wingspan. With the fuselage being made longer in the nose and taller to accommodate a fourth crew member, it was also considerably heavier. The larger bomb-bay was designed to house a semi-recessed torpedo, or it could carry an increased bomb load. Due to the increased weight the Blenheim's Bristol Mercury engines were to be replaced by the more powerful, sleeve valve, Bristol Perseus. It was soon determined that even with the Perseus, the Beaufort would be slower than the Blenheim and so a switch was made to the larger Bristol Taurus engine, also a sleeve valve design. For these engines, chief designer Roy Fedden developed special low-drag NACA cowlings which exhausted air through vertical slots flanking the nacelles under the wings. Air flow was controlled by adjustable flaps.
The basic structure, although similar to the Blenheim, introduced refinements such as the use of high-strength light alloy forgings and extrusions in place of high-tensile steel plates and angles; as a result the overall structural weight was lighter than that of the Blenheim.In addition, the wing centre section was inserted into the centre fuselage and the nacelle structure was an integral part of the ribs to which the main undercarriage was attached. Transport joints were used on the fuselage and wings: this allowed sub-contractors to manufacture the Beaufort in easily transportable sections, and was to be important when Australian production got underway. The Vickers main undercarriage units were similar to, but larger than those of the Blenheim and used hydraulic retraction with a cartridge operated emergency lowering system.
The first prototype rolled out of Filton in mid-1938. Problems immediately arose with the Taurus engines continually overheating during ground testing. New more conventional engine cowlings with circumferential cooling gills had to be designed and installed, delaying the first flight which took place on 15 October 1938. As flight testing progressed it was found that the large apron-type undercarriage doors, similar to those on the Blenheim, were causing the aircraft to yaw on landing. These doors were taken off for subsequent flights. On the second prototype and all production aircraft more conventional split doors, which left a small part of the tyres exposed when retracted, were used.
The results of high level bombing tests carried out at Boscombe Down at an altitude of 10,000 ft (3,000 m) and an airspeed of 238 mph (383 km/h) showed that the Beaufort was in the words of the test pilot: "An exceptionally poor bombing platform, being subject to an excessive and continuous roll which made determination of drift particularly difficult." After 1941, British Beauforts were fitted with semi-circular plates on the trailing edges of the upper wing behind the engine nacelles to smooth airflow and improve directional stability.
With Blenheim production taking priority and continued overheating of the Taurus engines there were delays in production, so while the bomber had first flown in October 1938 and should have been available almost immediately, it was not until November 1939 that production started in earnest. Several of the first production Beauforts were engaged in working-up trials and final service entry began in January 1940 with 22 Squadron of RAF Coastal Command.