The first atomic bombs had a low shelf life - 48 hours after assembly, a team of specialists had to recharge the batteries powering the electronic components (and there were only seven such teams as late as 1949). Even more modern designs have a life measured in decades. How would you do this in Test of Time? I suspect once the initial tech is researched, you can delay giving a second tech that "upgrades" the atomic bomber to a conventional plane using Leo's. If you removed the upgrading tech, could you pull this trick once more using the CityProduction trigger (as techumseh does in Sea Lion)?