two airframes, designated as the I-211 (I-21 version 1), began at the end of 1946, under extreme pressure by the Ministry of Aircraft Production to complete initial flight testing by 1 August 1947, to enable the aircraft to take part in the Aviation Day Flypast at Tushino on 18 August. While one of the two airframes initially produced was used for static testing, the other was fitted with Lyul'ka TR-1 turbojet engines because the intended TR-2 was not available. The engines were mounted in the middle of the wing, with the wing spars continued by banjo rings around the engines, much like those on the British Gloster Meteor fighter. Despite pressure from above, the I-211 was unable to participate in the Tushino display. Flight testing started in late 1947, but only six test flights had been carried out before the I-211 struck a pothole on landing, which collapsed the undercarriage. Even the limited amount of flight testing showed it to be faster than the Sukhoi Su-11 which also used the TR-1 engine. Repairs were carried out and the opportunity taken to replace the troublesome engines with imported Rolls-Royce Derwent V engines. Alekseyev took the opportunity to upgrade the aircraft to the I-215 configuration which had been in development since before the first flight of the I-211. The substitution of the centrifugal-flow Derwent for the axial-flow TR-1 forced the redesign and enlargement of the nacelles to accommodate the larger engine. Other modifications included increasing the fuel capacity from to , provision for a ranging radar in the nose, and better protection for the pilot. The armament was revised to the originally specified N-37 guns, although the ammunition capacity was marginally increased to 35 rounds per gun. Despite good results from flight testing, the I-215 lost out in production orders to the newer generation of swept-winged fighters. A second I-215 was built to an order from OKB-1 as the I-215D (dooblyor, second prototype), to evaluate the bicycle or tandem landing gear configuration. It used wider-diameter paired wheels in a tandem arrangement that retracted into the fuselage, along with small outrigger wheels under the engine nacelles which retracted into fairings. The main undercarriage of the I-215D also incorporated a kneeling feature which could increase the incidence of the aircraft by 3° to assist take-off. Trials with this undercarriage arrangement were successful and paved the way for its use in other Soviet aircraft, including