Work on the SR.45 commenced almost immediately upon receipt of the order. From an early stage in development, progress on the programme was subject to schedule overruns and higher than projected costs being incurred, leading to scepticism of the flying boat's viability within the House of Commons, which was being increasingly regarded as a high-risk initiative with unreliable costing. Specifically, designing and testing of the airframe, as well as the independently conducted development of the Proteus engine to power the SR.45, had been major contributors to the higher-than-expected costs.