Fixed price is notoriously difficult to estimate for: the technology is emerging, often state of the art and subject to major shifts in requirement. Best solution is to proceed cost-plus with risk sharing for feasibiliy and design-proving and fixed price for production on a per unit basis.
Airbus' first propeller and first military aircraft. It is hardly surprising that it hit problems. It is naive in the extreme to assume that it is like buying a newspaper. The customers often don't know what they want, and even if they do, it is often expressed in the language of the platform it replaces. Oh and they change their mind several times even during the feasibility stages. The mistake Airbus Mil made is lack of risk assessment and management.
hugel