The sustained efforts of Elie Carafoli and his team paid dividends when the first order for IAR 14 was finally placed. With production started and a small profit secured, Dipl.-Eng. Carafoli decided to try something new. The result was the I.A.R. 15. The in-line engine was abandoned and instead a radial powerplant was substituted. Accordingly, the front fuselage underwent a major redesign. The cross section was rounded and a NACA ring covered the 600 h.p. (447 kW) Gnome & Rhône 9Krse engine. With the new nine-cylinder radial the aircraft attained a top speed of 375 km/h at 4,000 m and could climb to 5,000 m in 8 minutes. The ceiling was raised to 10,500 m and an endurance of 600 km was possible. The I.A.R. 15 could intercept any contemporary major bomber type.
Alongside the new front fuselage section, other structural changes had been implemented as well. The cross section of the rear fuselage was enlarged and reinforced. The fuselage was a steel tube structure covered with dural forward of the cockpit and fabric aft. The tail was redesigned again, this time to a triangular shape and was also built of steel tube and dural covered. An aerodynamically improved single strut undercarriage featuring wheel spats was fitted near the wing roots and a small wheel replaced the previous tail ski. Instead of the crash-pylon featured on earlier models, a rounded knob appeared behind the pilot's head rest. The wings were rounded and shortened to 11.00 m, giving 19.00 m² total area, and were built around two dural spars with a mixture of wood and metal ribs. The open cockpit was aft of the wing trailing edge. The undercarriage had a wide track, with vertical wire braced and faired legs carrying spatted main wheels.
Power was from an I.A.R. 9KIc40 licence-built Gnome-Rhône 9K engine of 450 kW (600 hp) enclosed in a NACA cowling. In the first prototype this drove a two-bladed wooden airscrew, but later machines had three-bladed metal propellers.
Five prototypes were built. Tests showed that the IAR 15 was as fast as competing aircraft, chiefly the PZL P.11, but less manoeuvrable and no orders other were placed.