BERLIN/FRANKFURT, July 8 (Reuters) - Some of Germany's biggest corporations are weighing a joint initiative to boost investor confidence in Europe's top economy, two people familiar with the matter said, adding this included investment commitments and a push to attract private capital.

The initiative, which is spearheaded by groups including Siemens SIEGn.DE, Deutsche Bank DBKGn.DE and FGS Global, represents more than 30 of Germany's largest companies that have jointly committed around 300 billion euros ($352 billion) of investments to Germany until 2028, the sources said.

Most of this sum, which could still change, covers existing commitments these companies have already made, the sources said, while the hope is that the initiative will free up more funds from firms and investors.

RWE RWEG.DE, Rheinmetall RHMG.DE, SAP SAPG.DE and Volkswagen VOWG_p.DE could support the push, the people said, declining to be identified as the plans are not public and could still change.

Big international investors including Advent and KKR KKR.N are also expected to play a role in the initiative, which could be formalised later this month in cooperation with Germany's government, the sources said.

Deutsche Bank, RWE and FGS Global declined to comment.

SAP and Rheinmetall had no immediate comment.

($1 = 0.8519 euros)

(Reporting by Sarah Marsh, Christoph Steitz and Maria Martinez; Additional reporting by Alexander Huebner, Editing by Miranda Murray)

((christoph.steitz@thomsonreuters.com; +49 30 220 133 647;))