We report on–off pointed MAMBO observations at 1.2 mm of 61 Spitzer -selected star-forming galaxies from the Spitzer Wide Area Infrared Extragalactic Legacy survey ( SWIRE ) . The sources are selected on the basis of bright 24 \mu m fluxes ( f _ { 24 \mu m } > 0.4 mJy ) and of stellar dominated near-infrared spectral energy distributions in order to favor z \sim 2 starburst galaxies . The average 1.2 mm flux for the whole sample is 1.5 \pm 0.2 mJy . Our analysis focuses on 29 sources in the Lockman Hole field where the average 1.2 mm flux ( 1.9 \pm 0.3 mJy ) is higher than in other fields ( 1.1 \pm 0.2 mJy ) . The analysis of the sources multi-wavelength spectral energy distributions indicates that they are starburst galaxies with far-infrared luminosities from 10 ^ { 12 } to 10 ^ { 13.3 } L _ { \odot } , and stellar masses of \sim 0.2–6 \times 10 ^ { 11 } M _ { \odot } . Compared to sub-millimeter selected galaxies ( SMGs ) , the SWIRE-MAMBO sources are among those with the largest 24 \mu m/millimeter flux ratios . The origin of such large ratios is investigated by comparing the average mid-infrared spectra and the stacked far-infrared spectral energy distributions of the SWIRE-MAMBO sources and of SMGs . The mid-infrared spectra , available for a handful of sources , exhibit strong PAH features , and a warm dust continuum . The warm dust continuum contributes to \sim 34 % of the mid-infrared emission , and is likely associated with an AGN component . This constribution is consistent with what is found in SMGs . The large 24 \mu m/1.2 mm flux ratios are thus not due to AGN emission , but rather to enhanced PAH emission compared to SMGs . The analysis of the stacked far-infrared fluxes yields warmer dust temperatures than typically observed in SMGs . Our selection favors warm ultra-luminous infrared sources at high- z , a class of objects that is rarely found in SMG samples . Indeed SMGs are not common among bright 24 \mu m sources ( e.g . only about 20 % of SMGs have f _ { 24 \mu m } > 0.4 mJy ) . Our sample is the largest Spitzer -selected sample detected at millimeter wavelengths currently available .