Sounds like a job for Pelargoniums to me - these are commonly known as Geraniums, although they're not Geraniums at all, that's an entirely different plant. Check out Pelargonium hortorum - these have thousands of varieties, most of which are grown as ornamental flowers during summer round the world. Many have zonal markings on the leaves which vary in colour from orange to red to brown to black or white. In hotter, drier countries, they survive all year, but usually aren't in flower all that time. They come in upright forms, trailing (Ivy leaved pelargonium) or cascading (fairly short cascade, commonly sold as Cascade Geranium/Pelargonium). There's also one known as Regal Pelargonium, somewhat different in flower and leaf form - very attractive, but not quite as floriferous as the other types - they usually flower, then pause, then flower again. You should get 3 or 4 in your window box (because that's what you're describing), depending which varieties you choose. Drought resistant, they hate being waterlogged, but will need feeding and watering to keep them flowering well. Largely resistant to aphids and other sap feeding insects, though flowers can be spoiled by lots of rain.
One other little thing - I'd describe your planter/window box as being 'outside' the window rather than saying behind.