It looks like it might be Sonic Spinball. (Edit - Confirmed) (second edit) Here are two more sheets related to this title.
Definitely Spinball. The 16-bit box appears to be the original version, and the 8-bit after all the corrections were made. Four fingers + thumb, "delete round part of head", no curves on ears, points on spikes... it's all there. So at this stage they'd decided that they wanted to bring Sonic more in line with his (far superior) Japanese look. I'm so glad that someone said they had to get rid of the stupid mohawk look. But then they were still insistent on using their own design for Robotnik. Also interesting, as a bit of an aside, they eventually reinstated the exaggerated brow and curved ears for Generations. Love the Generations look. It only took them 20 years to nail Sonic's design.
I think this may have been related to the changes between the Master System/Game Gear Box Art and the Mega Drive/Genesis Box Art. This is the Master System/Game Geart Box Art (In spoiler tags due to size): Spoiler This is the Mega Drive Box Art (In spoiler tags due to size): Spoiler The changes mentioned on that sheet seem to correlate to the changes between them. EDIT: Ninja'd by Blue Blood
In-game I think it's a dome though, so I can understand the confusion. I haven't bothered to check though, I could be wrong.
Nah, it's always had a windscreen option. I say option because it appears to be able to retract to that position - compare these two screenshots to the opening movies from Sonic Unleashed and Sonic Lost World: http://www.youtube.com/watch?v=mqHxGoFERZw (jump forward to 1:50) http://www.youtube.com/watch?v=FEm9OYY9SvE (from 0:14 - and the thumbnail!)
Adding to what was previously said: In Sonic Spinball itself Robotnik doesn't use the eggmobile but instead this contraption that has a dome:
He's only doing the airtight protection in this boss because of his silent-but-deadly *Smell my socks* attack
Glad they made adjustments to Sonic. No offense to Greg Martin but I never liked that Americanized Sonic design. That period where Sega kept using Robotnik's AOSTH design though, it's too goofy even for Sonic. Glad that it never made its way into the actual games (besides Mean Bean Machine).
It would be interesting to read the actual reasons that SEGA gave him for changing these, even after the Mega Drive version had been released already. It's so weird that they let him change the designs so much from the Japanese versions, for so many game covers. EDIT: Now that I think of it, it might have to do with SEGA of Japan taking more control over their overseas divisions, after the Mega-CD crisis.
...anyways, just curious, where did that corrections sheet come from exactly? For all I know, some random kid could have drawn that up 2 days ago.
Man, that's really cool. I just took those corrections to the Sonic 1 box art: Quick and rough, but imagine how cool it would've been if it was originally done more on model.
Do we know why did Greg Martin used a 3 finger hand on Sonic Spinball? I know that's a norm in classic animation, but all his other Sonic artworks have 4 fingers?
This and the 8-bit Spin all art make it so clear how it wasn't so much Martins artistic talent that was poor, but rather just the awful design he was using. Wow.
Another point is that Martin didn't illustrate the Sonic 1 Genesis cover. When he took over as cover illustrator the NA Sonic image was already established.