Within a short span of time, the Bethmann bank developed into one of Frankfurt's leading (Christian-owned) banks, on a scale comparable only to its younger rival, the House of Rothschild. The bank's fortunes began to rise in 1754 based on its business in imperial, princely and municipal bonds and skyrocketed from 1778, thanks to the bank's innovation of breaking the Austrian emperor's borrowing down into "sub-bonds" (Partialobligationen) at 1000 gulden each offered to the public, which made them tradeable in secondary markets. This transformed the bank from a lender to an underwriter of bond issues. At one point, the profits of Gebrüder Bethmann exceeded those of all its Frankfurt competitors together, and it ranked first among all German banks.
In spite of this new offensive in favor of the House of Rothschild, the business was entrusted to Bethmann, on the ground that the property of that house was 'well known to be so great as to require no other security than a duly binding contract or debenture.' This was a triumph of Bethmann over Rothschild, and it was because the importance and financial greatness of the firm of Rothschild, which at that time  was still of very recent date, was not appreciated at Vienna, and even Barbier had more faith in the old Christian firm of Bethmann than in the upstart Jewish firm.
Paul Johnson (1988)
What happened was this. Until the beginning of the revolutionary wars in France, in the mid-1790s, European merchant banking was dominated by non-Jews: the Barings of London, the Hopes of Amsterdam, and the Gebrüder Bethmann of Frankfurt.
Founding of Frankfurter Bank, together with Joh. Goll & Söhne, Philipp Nikolaus Schmidt, D. & J. de Neufville, Philipp Donner, W. Gansland & Sohn, and B.H. Goldschmidt.
Eiffel tower is built in Paris with financing provided by Bethmann bank. (In a repeat of sorts, the Singapore Flyer would be built based on S$240 million financing provided by Delbrück Bethmann Maffei and one other German bank, and the bank also co-financed the Great Orlando Wheel project.)
In 1763, when Wolfgang Amadeus Mozart and his family were visiting Paris, a letter of recommendation penned by a wife of either Johann Philipp or Simon Moritz Bethmann and addressed to Baron de Grimm served as an effective door opener, as Leopold Mozart wrote afterward.
When Johann Wolfgang Goethe traveled to Italy in 1768, he was using a bill of exchange payable by a Roman banker and drawn on the Bethmann bank, which had issued the letter to his pseudonym of Möller, not knowing the true identity of the payee.
^In 1818, Baring Brothers and Hope & Co. joined Gebrüder Bethmann (and Geymüller of Vienna as well as Parish of Hamburg, two banks that would later succumb to competition from the Rothschilds) in taking over the Austrian emperor's 50 million so-called Metalliques bond issue (Achterberg, p.44).