Blackburn Iris The Blackburn Iris was a British three - engined biplane flying boat of the 1920s . Although only five Irises were built , it was used as a long - ranged maritime reconnaissance aircraft by the Royal Air Force , where it equipped a squadron for 4 years , being used to carry out a number of notable long - distance flights . The final version of the Iris , the Iris Mark V was developed into the aircraft that replaced it in Squadron service , the Blackburn Perth . Development and design . 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 J . B . Rennie , who had previously 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 dosal gun positions mounting Lewis gun s on Scarff ring s , 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 . Operational history . On 4 February 1928 , a contract was placed for three Iris III aircraft , similar to the Iris II but with fabric covered metal wings . The Iris II , meanwhile , continued in use , setting out on another long - range cruise on 27 September , carrying Sir Philip Sassoon , the Under - Secretary of State for Air and Air Commodore Sir Arthur Longmore on a tour of RAF Stations of the Mediterranean and Middle East . It reached Karachi on 14 October , finally returning to RAF Calshot on 14 November , having flown a total distance of 11 , 360 mi ( 18 , 290 km ) with a flight time of 125 hr 5 min . The first Iris III flew on 21 November 1929 , with the three Iris IIIs equipping No . 209 Squadron which reformed at RAF Mount Batten , Plymouth in January 1930 . 209 Squadron continued the pattern of long - range flights carried out by the Iris II , with one Iris visiting Reykjavík in June 1930 to celebrate the 1 , 000th anniversary of the Icelandic Althing ( parliament ) , and another making the first crossing of the Bay of Biscay by flying boat when visiting Lisbon in August 1930 . The first Iris III was destroyed in a fatal crash on 4 February 1931 , resulting in a replacement being ordered . Although still an Iris III , this had a number of changes , being fitted with provision to carry a COW 37 mm gun in its bow . The weight of the Iris had grown considerably since the Iris I , and it was decided to replace the Iris III ' s Condors with more powerful ( 825 hp ( 615 kW ) Rolls - Royce Buzzard engines to restore performance and improve reliability , with the three re - engined aircraft re - entering service in 1932 as the Iris V . The Iris II was also re - engined , being fitted with three 800 hp Armstrong Siddeley Panther radial engines , with the centre engine in a pusher configuration to become the Iris IV . The Iris Vs were replaced in squadron service in 1934 by the Blackburn Perth , four very similar Buzzard - powered aircraft closely based on the Iris . One of the Iris Vs was converted for use as a testbed for the Napier Culverin , a licence built Junkers Jumo 204 diesel engine , flying in this form in June 1937 and continuing flight trials until April 1938 .