On 14 May 1939, the first S.29, which had by this point received the service name "Stirling" after the Scottish city, performed its first flight. The first prototype was outfitted with four Bristol Hercules II radial engines, and was reported as having satisfactory handling in its two months of flying. However, the entire programme suffered a setback when the first prototype suffered severe damage and was written off as a result of a landing accident, in which one of the brakes locked, causing the aircraft to slew off the runway and the landing gear to collapse. A resulting redesign of the undercarriage led to substantially stronger and heavier struts being installed upon the second prototype. On 3 December 1939, the second prototype made its maiden flight. During its first sortie, one of the engines failed on takeoff but the second prototype managed to land with relative ease.