The earliest example I can find is the 1668 work by John Wilkins: "An Essay towards a Real Character, and a Philosophical Language"¹ , where he proposed a universal language and a decimal system of measure not unlike the modern metric system.
Wilkins was well-known and regarded (having headed colleges at both Oxford and Cambridge, and helped found the Royal Society), and famous in later ages, and Philosophical Language appears to be his best-known work.
Using the Google NGram for "An essay towards" from 1650-1850 to investigate whether Wilkins' essay had a material impact on later authors, it turns out Philosophical Language doesn't show up at all (possibly too early). However, there is a large spike in 1713 as well as 1738; the former is almost certainly Bishop Berkley's An Essay Towards a New Theory of Vision. Googling suggests the latter is something called An Essay Towards the Character of the Late Chimpanzee, who Died Feb. 23, though there's no way to be sure.
Anyway, Berkley would have surely known of Wilkins, and may have borrowed the "Essay towards" construct from him. And while not large as the 1738 spike (nor the complete step function of 1713), there are spikes in the usage of Towards... every decade or two, with each generation almost certainly influencing the next.
So, given his influence and legacy, it is possible (though unknown) that Wilkins' title inspired the trend which continues today.
¹ And its simultaneously-published companion work "Alphabetical Dictionary", subtitled "Towards a comprehensive, and systematically defined, lexicon".