Would some joint initiative have inevitable arisen?
Keep Canada in the Anglo/Imperial sphere of influence, which is an entire problem in itself, so it won't turn to America.
It is inevitable that, as Canada and the US grow stronger economically and it moves away from the British sphere of influence,* since the simple size of the Empire does pose significant problems to both trade and security, that security of North American trade becomes a concern. Especially in the
context of the Cold War.
*Prime Minister W.L.M. King was all about nation unity following WW1, and he believed that any Canadian commitment to the Empire would damage this unity as seen in WW1. So he was motivated in keeping Canada out of Imperial issues. So, he was more inclined to strengthening American-Canadian relations, than to the Empire. And, FDR helped out a lot as well in the 1930's.
Right before WW2, FDR come to Canada and gave a speech saying that, the USA would back and help out Canada if it became threatened by a foreign power, say Nazi Germany. This starts the process of Canadian-USA cooperation, which deepened during WW2, and will continue to evolve until we see NORAD.
Geography is a huge factor. As well as larger global issues (WW2, Cold War, etc), NORAD does seem rather inevitable. I only see a big and early POD necessary, or
not having King being PM for so long could help so we have a Prime Minister who want Canada to be a central part of the Empire. Imperial Federation anyone?