Without Love (1945) on Netflix

In WWII Pat Jamieson is a scientist working, with Government support, on a high-altitude oxygen mask for fighter pilots. But he has nowhere to conduct his research in secret until he meets Jamie Rowan, a woman with an unused house with a scientist's basement. Jamie has no hopes of marrying for love (and neither does Pat) but Jamie wants to help the war effort and she likes this quirky scientist and his dog, so to satisfy the proprieties they agree on a business arrangement: a marriage of convenience and partnership. They happily work on oxygen mixes instead of honeymooning. But as the footing of their relationship begins warm up, Jamie is courted by another man and the old flame that broke Pat's heart is back in his life. It will take a sleepwalking ruse, dodging in and out of doors, and a working oxygen mask to get them together again.