Flat windshield on aircraft canopies...

[Boris]

For a very long time a majority of aircraft had a flat windshield part at the front of the canopy. This can be seen from the Spitfire and Mustang all the way through to at least the 70s, in US, British, French and Soviet fighters.

 

Here's what I mean:

 

 

I've always wondered why this is. It obscures a lot of the view to the front.

Was it for structural integrity?

However, there were contemporary aircraft that had cleaner bubble canopies, such as the T-38 that was introduced in 1961.