Challenge Based Learning (CBL) is an educational approach that has gained popularity in response to the need for authentic learning environments. While the CBL literature is predominantly focused on cases of pedagogical implementations, the actual processes by which students develop CBL projects remain under-investigated. This shortitudinal study seeks to examine the phases of CBL project development and associated process behaviours at group level, as they unfold. The participants are 6 interdisciplinary student groups totalling 22 students enrolled in a first-year course on ethics and data analytics. Data was collected weekly throughout 10 weeks via reflective diaries (n = 15 students) and observation of course sessions (n = 22) and is complemented by interviews after course completion (n = 15). The data was subject to a thematic trajectory analysis. The study identifies 7 distinct phases in the temporal structure of a CBL project: gaining client know-how, articulating a problem, mapping the problem context, setting the aim, proposing an action path, testing and evaluating it, and implementing the solution. The article concludes with recommendations for further research into CBL project development processes, which may support the growing adoption of real-life interdisciplinary projects in engineering education.