Disclaimer: I work at flightfox.com.
What I think you’re asking for already exists. You didn’t mention price, which makes a big difference, but price aside, route data is available in both raw and GUI form. Check out flightconnections.com for a GUI tool.
Price is what consumers care about and without a perfectly efficient (or regulated or monopolized) market, there will never be a single source of all prices for all routes. This doesn’t exist for food, clothing or many genuine commodities.
Someone else replied with a link to ITAs white-paper that discusses the difficulty of the problem you’ve described. But, that only considers cash flights; what about fast trains, frequent flier miles, buying and using points, credit card bonuses, best rate guarantees, combining low cost-carriers, charter airlines, cargo flights, consolidator deals, local deals, company benefits, etc.
It’s a huge market that feels like it should be commoditized but is actually quite differentiated with significant barriers to entry.
My personal experience at Flightfox is overseeing the booking of travel for more than 60,000 people using travel websites rather than traditional GDSs (Global Distribution Systems - what travel agencies and online travel agents use). Our team has used most of the websites mentioned here and never relies on just one.
Most coders here could build a simple tool to make their searches much more effective and efficient:
* Accept date/route/class input, like “10SEP DEN-FRA first”
* Open tabs to 5-10 of the best and most varied travel websites
* Skim the results and book
Doing this rather than sticking to a single site will yield much better results and make your search more efficient. The largest OTAs have real proprietary deals, the best of the aggregators search the best search engines, and the best search engines combine the most airlines (even LCCs) in creative ways.
If I had to choose 5 flight websites for such a tool: Kayak, Google, Orbitz, Skyscanner and Momondo.
Given this, you may wonder what’s the point of Flightfox. We started with a pure focus on travel hacks (much more advanced than searching 5 OTAs), but we now do our best work managing travel for businesses. For consumers, we can only use one skill: saving money. For businesses, we can become their entire travel department and put everything we know into practice. Not just saving many $1000s/month, but helping with finance strategy, loyalty programs, upgrades, 24/7 emergency support, super-cheap business/first, etc.