In 1924, the British Air Ministry issued Specification R.14/24 for a long-range reconnaissance flying boat for the Royal Air Force. To meet this requirement, Blackburn Aircraft proposed the R.B.1 (Reconnaissance Biplane 1), designed by Major John Douglas Rennie, who as Chief Technical Officer worked with John Cyril Porte at the Seaplane Experimental Station, Felixstowe.
The R.B.1 was a three-engined, three-bay biplane. The equal-span wings were of mixed wood-and-metal construction, with ailerons fitted to both upper and lower wings and floats fitted under the wingtips, while the aircraft had a large biplane tail (with a span of 30 ft (9.14 m) with three fins and rudders. The aircraft's hull had a wooden structure covered in plywood, with a vee-bottom with two steps to give good water handling. Three 650 hp (485 kW) Rolls-Royce Condor III water-cooled V12 engines driving four-bladed propellers were mounted in individual nacelles between the wings. It carried a crew of five, with two pilots sitting side-by side in a cockpit forward of the wings, with nose and dorsal gun positions mounting Lewis guns on Scarff rings, with provision for a further two guns which could be operated through portholes in the rear fuselage. Bomb racks under the wings could carry up to 1,040 lb (470 kg) of bombs.
The prototype R.B.1, with the designation Iris I, and with the serial number N185, made its maiden flight from Blackburn's factory at Brough on 19 June 1926, being delivered to the Marine Aircraft Experimental Establishment at Felixstowe the next day, being fully tested during July and August. Wooden hulls were prone to soaking up large quantities of water (which could amount to several hundred pounds in additional weight) when kept afloat for long periods of time, so Rennie designed an all-metal hull for the Iris, constructed of duralumin before the Iris first flew. N185 returned to Brough in March 1927 when it was fitted with the new metal hull, together with more powerful engines and an additional gunner's position in the tail, becoming the Iris II.
On 12 August 1927, shortly after being redelivered, the Iris II started, along with the prototype Short Singapore I, an experimental metal-hulled Supermarine Southampton, and the prototype wooden hulled Saunders Valkyrie (a direct competitor to the Iris), a 3,000 mi (4,800 km) tour of Scandinavia and the Baltic. The Iris performed well on the tour, particularly compared to the Valkyrie, which suffered much heavier water soakage than expected as well as engine problems, and the Air Ministry issued Specification R.31/27 for an improved version of the Iris, to act as a long range supplement to the smaller Southampton.
- R.B.1 / Iris I : Prototype, wooden hull and mixed construction wings, powered by three 650 hp (485 kW) Rolls-Royce Condor III engines. One built.
- R.B.1A / Iris II : The Iris I with a new, all metal hull and three 675 hp (503 kW) Rolls-Royce Condor IIIA inline piston engines.
- R.B.1B / Iris III : Five-seat long-range maritime reconnaissance flying boat for the Royal Air Force. Metal hull and wings. Powered by three 675 hp (503 kW) Rolls-Royce Condor IIIB inline piston engines. Four built.
- R.B.1C / Iris IV : Conversion of the Iris II with three 800 hp (600 kW) Armstrong Siddeley Leopard III radial piston engines.
- R.B.1D / Iris V : This was the final variant. Three Iris Mk IIIs were fitted with 825 hp (615 kW) Rolls-Royce Buzzard IIMS piston engines.