At the risk of making comments unworthy of this august body of scholars, I would like to say that "development economics" developed (no pun indented) as a sub-field of economics in distinct ways in the fifties, sixties and seventies and continues to evolve as we know from its adoption of the experimental method. The term may have ancient origins, and the classical economists appropriately concentrated on growth and capital accumulation, but that does not contradict the fact that a body of theory and practice emerged after the break-up of colonial empires. I remember the excitement we felt in studying development theory in India way back when, even embracing central planning as an antidote to reckless emulation of industrialized societies. [By the way, recent liberalization there has indeed led to rapid growth, but also increasing and unconscionable income inequalities.] I think the original query about the idea of development and its origins is very valid for historians of economic thought.
Best, Sumitra