Why has European integration affected some of Europe’s defence firms more than others? Specifically, what explains the co-existence of national, transnational and European champions in this industry? This article develops answers to this question from two complementary angles. First, through examining the business models and turnover of the four largest companies in Europe (BAe Systems, Airbus, Thales, and Leonardo), it shows that firms who mostly produce military goods are less likely to undergo strong European integration. Second, using an original database on the social backgrounds of these firms’ board members, two further hypotheses are tested. Using data on higher education and careers, on the one hand we show that the relationship of board members to their respective state varies from close (Thales and to some extent Airbus) to distant (BAe Systems and Leonardo). On the other, our data reveals that when the careers of these actors are frequently internationalised, this correlates to either strong European integration at the level of the firm (Airbus and Thales) or, alternatively, strong Transatlanticism (BAe Systems or Leonardo). The article as a whole thus both opens up new avenues for research on the defence industry, whilst adding political economy and sociological dimensions to existing scholarship on differentiated European integration.