About the paperWhether generic 'silver bullet' solutions can address complex development problems has been debated for many years. The 'grand challenge' extends the idea of the silver bullet in ways that speak to a goal-driven, global development agenda and a new generation of private philanthropists -or 'philanthro-capitalists' seeking to apply business methods to 'strategic' giving. These developments raise new Sustainability challenges, explored in this paper, drawing on examples from the health and agriculture sectors. Biofortification research funded by the Bill and Melinda Gates Foundation provides a detailed, illustrative case of how these ideas can reduce space to debate directionality and accountability. Imperatives towards rapid 'scaling up' infer homogenous populations and overlook patterns of diversity and distributional concerns; transforming complex and diverse needs into 'demand' for pre-defined technical solutions. This paper asks if the potential exists for a reinvigorated philanthropic sector to play a different role, and turn its power and resources towards learning processes that recognise diversity and use this to reshape programme design.