The Boeing sales machine swivels on Thursday from London to Chicago.

Boeing and United Airlines have called a press conference for 9 a.m. central in Chicago to announce a large order that's been expected for some time.

United CEO Jeff Smisek will appear with Boeing CEO Jim McNerney, along with Ray Conner, the new CEO of Boeing Commercial Airplanes.

Conner must have jetted to Chicago Wednesday evening. Earlier today, I saw him pass through the Boeing media chalet at the Farnborough Air Show here in London.

The news will significantly add to Boeing's sales tally from the Air Show.

A reliable source familiar with the deal confirmed that the size of the order is what has long been rumored.

United will place a firm order for 100 of Boeing's single-aisle 737s, most of them the forthcoming model with new fuel-efficient engines, the 737 MAX.

The airline will have options to take 100 more.

The list price for that many 737s depends on the model mix but could be as high as $10 billion. The actual purchase price, based on aircraft market pricing data from jet valuation firm Avitas, should be just over $5 billion.

In the last week, Boeing has landed 98 firm orders for the MAX, 23 from Virgin Australia and 75 from Air Lease Corp.

It has also disclosed previous commitments, not yet firm orders, for 100 MAXs from leasing company GECAS, 20 MAXs from Kuwaiti lessor ALAFCO, and a mix of 15 MAXs and 10 current generation models 737NGs from Irish lessor Avolon.